If I think along the lines of Steins;Gate, then Mami's death is a basic World Line Convergence point in the Madoka attractor field universe. So I'm also betting that that's Mami's blood.
Mami with her head Charlotte in mouth but do to her strength in this timeline her neck broke Charlottes teeth. For extra action, a fist to whatever is Charlottes throat while Mami still has her head inside her mouth causing this blood splatter.
I'm actually hoping Mami dies. That would be such a plot twist. It would completely change the tone of this comic. And it would be all the more of a twist because clearly, no one expects it, everybody just thinks it's too obvious. It's like, I don't know, a double plot twist. So meta.