Hey
I've never built a shifter/druid based blackstaff toon, but have built a fighter/wizard melee version and it is one of those builds that I posted on the forums under the speciality builds section:
http://www.gondorvsmordor.com/gvm2/forum/index.php?topic=1466.0I suppose the only disadvantage I can think of, as with any wizard based druid/shifter build, is that once it wears off you have to deshift, but extended spell will help with that.
General thoughts on Blackstaff is that it is a great tool - my build specifically gets 61% vunerability effect which you do notice, and any party member will gain from the effect too.
The only drawback of the spell itself that I can remember is that the on-hit effect has a timer, probably to stop it becoming a spammable effect. By this I mean if you bane someone (causing them to get the vunerabilities and glow red) and then that target is restored via one of the restoration spells (not sure which level of the spell works lesser/regular/greater), the effect is removed from the target. The next bane success requires an amount of time to pass before it can be re-applied to that target (I don't recall what that time is). So killing off clerics or anything with restoration spells first is probably a good idea incase they start casting restorations on all of their friendly NPCs.
Fairly recently (in terms of overall mod lifespan) I introduced a new Quarterstaff into the mod which is a very nice weapon for blackstaff toons. It's also a lot easier to get that the one from Barad-dur and arguably nearly just as good.