Blue
Ravenous Bugblatter Beast of Traal
I think the point Lancelot is trying to make regarding Barkskin is that the spell states the affected creature gains the AC bonus due to its skin becoming hardened like a bark. However, when a druid turns into a bear, its hardened skin is now replaced by the skin of a brown bear, therefore making the spell ineffective while in bear form. However, being able to hold concentration means that if the druid was to loses its bear form for any reason and revert back to itself, the Barkskin spell is still in effect and the AC bonus still applies in the normal form.
This doesn't quite match the spell.
The skin gets hardened, and stay hardened as long as concentration is maintained. It doesn't matter that the skin changes to bear skin - the concentration is maintained so the bear skin becomes barklike.
The spell clearly ends if concentration is broken, and clearly doesn't end with concentration still up and duration unexpired. A ruling that it goes into remission (but yet still needs concentration maintained) may be an applicable house rule for your group, but since that type of behavior isn't seen anywhere else in the rules I shouldn't assume it's the default or one of the defaults for a discussion of wildshape power.
Druid is the creature touched, doesn't matter if it's their own barkskin, a wizard's invisibility, or other spell that can target them. The spell is not instantaneous, it's ongoing for the duration/concentration. (Try the same narrative assuming the invisibility - seems fairly wild to assume their reappear as a bear and then could disappear again if they ended their wildshape.)