What materia and stats I should go for on dark knight?

[VenKitsune]

And need I to remind you that Skill Speed only affects Auto attack damage, DoT damage, and weaponskill recast, and does not affect your actual damage output. It even says so in that stat tooltip for skill speed.

You need Skill speed in 99% of situations in order to hit all 5 GCD's during Delerium, and all 5 GCD's during Blood weapon. If you lack Sks and suddenly can only hit 4, you've dropped potency as a result of either less overall mp regen, or in the form of dropping a bloodspiller. Its as simple as that, no ammount of extra crit/dhit chance will make up for missing an entire attack once every 90 seconds.