People are pretty annoyed by Morgana bot lane because of her ability to deny plays once every 26 seconds.
What if her shield could only target herself? This would solidify her position as a mid laner, but she'd still be useful as a support in a manner similar to Lux, Velkoz and Brand.
Her shield can still protect allies from projectiles. You merely blackshield yourself and walk in front of the attack. Which is the optimal way to play Morgana right now anyway, but it makes her positioning more important.
If Blackshield can only target Morgana herself then instead of a support mage with a ton of denial, she just becomes a mage with an "escape" similar to every champion which has a dash or other "get out of jail free card". This is balanced because her damage is A LOT lower than other mages. Oh, and it fits her original champion design well. The purpose of black shield is to enable Morgana to use her ultimate. So why not just make it only self targetable?
Of course actual black shield numbers may need to be tweaked for the significant loss in utility.