Here's hoping you aren't actually sad right now. :c
At any rate, I was wondering if that is a shading error in the top-right there. On the, uh, middle... tufty... part (expert terminology). I checked the speedpaint and it looked fine at the end of the red part, so maybe a layer got messed up when you turned it blue or something, I dunno.
I don't think it's a bloodstain and it's unnatural for a piece of cell shading left alone surrounded by blurred shading, so I think it's a mistake. And Dotty mentioned that she/he saw it in the speedpaint video too.