DRK - Multiple bugs.

ISP: Comcast
Type of Internet Connection: Cable
Internet Connection Speed: 20Mbit/s
Date & Time: 6/30/2015 @ 11:53 AM EDT
Frequency: Always
Character Name: Yishunsen Mol
Race: Au Ra Male
World: Balmung
Class/Level: Dark Knight 60
Area and Coordinates: Mist (9,10)
Party or Solo: Solo (Party to test)
NPC Name: None
Monster Name: None
In-Game Time: 9:45 AM

Steps:
1) Have a friend cast regen on you, or be tagged by enemy poison. MP is drained based off Heal over Time effects or poison ticks (thus resulting in faster MP drain than intended).
2) Using Dark Arts with Dark Passenger only blinds 1 enemy monster in a group.
3) Using Dark Arts with Abyssal Drain only drains the health of 1 enemy monster in a group.

Further editing on viewed bugs:

Mana is drained when buffs are used and then eventually fall off. This does not seem like intended behavior.

ISP: Plusnet
Type of Internet Connection: Fiber
Internet Connection Speed: 70Mbit/s
Date & Time: 6/30/2015 @ 11:53 AM EDT
Frequency: Always
Character Name: Farah Starmaim
Race: Miqo'te Female
World: Coeurl
Class/Level: Dark Knight 54
Area and Coordinates: Everywhere
Party or Solo: Both
NPC Name: None
Monster Name: None
In-Game Time: Any

Above bugs are the same for me. I will also add that any buffs applied to a Drk drain their mana, not just ones cast on themselves. (For example, Astrologian card buffs.) MP is drained both on application and expiration, causing massively accelerated MP drain.

ISP: Verizon
Type of Internet Connection: Tether
Internet Connection Speed: 50Mbit/s
Date & Time: 6/30/2015 @ 11:38 AM CST
Frequency: Always
Character Name: Felorr Bojangles
Race: Au Ra Female
World: Leviathan
Class/Level: Dark Knight 60
Area and Coordinates: Everywhere
Party or Solo: Both
NPC Name: None
Monster Name: None
In-Game Time: Any

Steps:
1) Activate Darkside
2) Acquire a buff from another player
3) Watch as MP is drained by buff application and removal
4) HP regens will drain MP with each tick

ISP: Bell
Type of Internet Connection: Fiber
Internet Connection Speed: 50Mbit/s
Date & Time: 6/30/2015 @ 12:45 PM EDT
Frequency: Always
Character Name: Eej Ette
Race: Au Ra Female
World: Excalibur
Class/Level: Dark Knight 58
Area and Coordinates: Coerthas Western Highlands (31,38)
Party or Solo: Both
NPC Name: None
Monster Name: None
In-Game Time: Any

There is no actual mana loss with this bug. It is purely a client side visual bug. Gaining a buff seems to make the client think that a Darkside mana tick has occurred and you lose mana equal to two Darkside ticks. In combat, you don't normally see upwards mana ticks, just a steady decrease in mana. If you use a buff, your MP bar gets hit by a double strength Darkside tick and the next Server tick brings you back up in mana by a single Darkside tick (so it pretends nothing happened).

However since the Client is telling you if you have enough MP to cast something, receiving a buff in between server ticks may temporarily drop your mana low enough to prevent you from using a mana ability.

E: More testing.

It seems like any buff, change in MP bar or Heal Over Time tick makes the client think that your MP has gone down by two Darkside ticks. At 58, I lose exactly 100MP per server tick with Darkside up in combat. Whenever I use an ability or a spell like Unmend, I lose 208MP. In the case of Unmend it's even worse, because it appears to cost 278MP + 208MP, which then gets fixed in the next server MP tick. The worst case is with a Heal over Time. With Aspected Benefic regen on my character, I lose 208 MP every time it ticks a heal which 108MP is then refunded me at every server tick (because I only lose 100MP per tick with Darkside at my level). It also explains why it seems that we naturally regenerate some MP in combat while under Darkside. We don't! Those are just the server refunds from us hitting so many abilities inbetween ticks.

ISP: PTV Telecom
Type of Internet Connection: Fiber
Internet Connection Speed: 100Mbit/s
Date & Time: 6/30/2015 @ 19:00 GMT+2
Frequency: Always
Character Name: Lexington Xer'ath
Race: Au Ra Male
World: Ragnarok
Class/Level: Dark Knight 60
Area and Coordinates: Everywhere
Party or Solo: Both
NPC Name: None
Monster Name: None
In-Game Time: Any

/ Confirmed Bug \
Using Dark Arts with Dark Passenger only blinds 1 enemy monster in a group.
Using Dark Arts with Abyssal Drain only drains the health of 1 enemy monster in a group.

About MP "Inusual" drain:
EDIT: Testing right now with a DRK 52 and when i use regen on him or any other buff i dont see him losing more MP than usual Dark Side do
EDIT 2: Second test with a WHM friend and myself DRK60 in Idyllshirne and inside dungeons he casting regen and more buff on me and i dont lose mp quicker than before
EDIT 3: With dx11 i dont have this inusual mp drain tick but i find i have this issue with dx9. Its seems a client-side problem?

ISP: Service Electric
Type of Internet Connection: Tether
Internet Connection Speed: 50Mbit/s
Date & Time: 6/30/2015 @ 1:48 PM EST
Frequency: Always
Character Name: Stefan Delumiere
Race: Hyur Midlander Male
World: Balmung
Class/Level: Dark Knight 60
Area and Coordinates: Everywhere
Party or Solo: Both
NPC Name: None
Monster Name: None
In-Game Time: Any

Steps:
1) Activate Darkside
2) Acquire a buff from another player
3) Watch as MP is drained by buff application and removal
4) HP regens will drain MP with each tick

ISP: Telecom Italia
Type of Internet Connection: Fiber
Internet Connection Speed: 30Mbit/s
Date & Time: 6/30/2015 @ 19:00 GMT+2
Frequency: Always
Character Name: Selnia Vicker
Race: Au Ra Female
World: Moogle
Class/Level: Dark Knight 60
Area and Coordinates: Everywhere
Party or Solo: Both
NPC Name: None
Monster Name: None
In-Game Time: Any

Can confirm the above bugs:
1. Casting Dark Passenger under the effect of Dark Arts only applies the added effect on the monster you're attacking, other monsters will receive non-boosted Dark Passenger damage and no Blind effect is applied.
2. Casting Abyssal Drain under the effect of Dark Arts only gives back health from 1 enemy, other enemies only take damage without giving back health.

ISP: Virgin Media
Type of Internet Connection: Fiber
Internet Connection Speed: 30Mbit/s
Date & Time: 6/30/2015 @ 21:09 BST
Frequency: Always
Character Name: Zeke Vardenfell
Race: Hyur Highlander Female
World: Odin
Class/Level: Dark Knight 54
Area and Coordinates: Everywhere
Party or Solo: Both
NPC Name: None
Monster Name: None
In-Game Time: Any

Steps:
1) Activate Darkside
2) Acquire a buff from another player
3) Watch as MP is drained by buff application and removal
4) HP regens will drain MP with each tick

Just chiming in I'm also seeing the MP usage when buffed form another party member in darkside. This also happens when i receive stacks of debuffs like poison.

ISP: AT&T Uverse
Type of Internet Connection: Tether
Internet Connection Speed: 18 Mbit/s
Date & Time: 6/30/2015 @ 20:00 EST
Frequency: Always
Character Name: Docfiord Fowling
Race: Au Ra / Xaela / Female
World: Malboro
Class/Level: Dark Knight 60
Area and Coordinates: Everywhere
Party or Solo: Both
NPC Name: None
Monster Name: None
In-Game Time: Any

Steps:
1) Activate Darkside
2) Acquire a buff from another player
3) Watch as MP is drained by buff application and removal
4) HP regens will drain MP with each tick