Morgana's Spell shield is OP. Many people will say that the reason for it being OP is that it absorbs so much damage, but I see that as working as intended. The problem I have with Black Shield is the fact that it can get your or your teammates out of any crowd control. If you're slowed or even rooted all you have to do is cast this on yourself and the effect is instantly gone. If your teammate is stunned or feared, casting Black Shield on them will cancel the effect immediately. Basically it functions like the Cleanse summoner spell and also blocks damage and also is useable on teammates.
I have no problem with spell shields blocking damage and any crowd control effects, but they should be balanced in some other way. Sivir, for example, can block all damage and any other effects, but she can only cast her shield on herself and it only blocks one ability, whereas Black Shield blocks for the entire duration.
Bottom line: if you want to negate crowd control, get Banshee's Veil or Mercury Boots. Black Shield is perfectly fine for blocking damage, which it should, but is utterly broken in terms of canceling crown control and giving your team a massive advantage in any fight.