I got it!
Post-70 BLU is going to be retooled to be a melee DPS, because that's what it usually is.

Yoshi-P's shirts depict Grim Reapers. Reapers are charged with going around and COLLECTING souls, in a similar fashion that BLUs go around COLLECTING souls of their foes to learn/use their spells.

FYI, I'm not serious and I'm only joking.