Just a random thought I had about the BLU RNG abilities, tie them to a Job Gauge.
Call it something like the "Doom Gauge" or something, and have it fill up every time you cast a spell. Change all of the rng skills (Doom, Tail Screw, Missle) to OGCDs with cast times that cost "Doom Gauge" to cast, with Tail Screw and Doom with CD as well. Then Make is so when you cast these skills and they fail they do potency instead, Missle doing the weakest potency at the weakest doom gauge cost and no CD, and have Tail Screw and Doom share a cooldown, with Tailscrew doing a fixed potency (250 or something if it fails) and have Doom have a scaling potency based on the health remaining on the enemy, (I.E when the boss is 50% or lower it'll be better to use doom over screw). That way you keep the chance of these skills working on enemies in dungeons that are weak to them, but you naturally nerf the frequency at which it happens, so it just becomes a cool moment if it does happen. Doom would also add some synergy in the BLU kit as it would get buffed by bristle if the enemy was on low enough health.