Definitely, a nesting mother. Be glad it's a blackbird and not a crow: those things go for blood.

my guess is she is protecting her nest that has eggs or babies. I have a blackbird that lives in a tree that is on a path I jog, and every spring that damn bird will buzz my head trying to scare me off.

This is an A+ example of what would send me off the edge. I walk around NYC in constant fear that this will happen to me.