It's tempting to draw parallels between the two types of performance: they both rely on exaggerated stereotypes, they're both one group impersonating another group, and they both come from a long historical tradition that goes back to Shakespeare (and beyond). But that is where the similarities end.
blackface is inherently racist. You can't take the racism out of it. Drag, on the other hand, you can easily separate from misogyny.
Blackface is a lie about a minority group, and intended to ridicule and dehumanize blacks while promoting racism. Blackface is used agasint the person where drag if focused on the stereotypes and cultural expectations. Drag is an expression of yourself, whereas blackface is an attack on someone else; blackface reinforces an imbalance of power, whereas drag disassembles an imbalance of power