I've had this problem with transmapped hair. Try increasing the Max Transparent Bounces. The default is too low, and often people here recommend a setting that's even lower (to speed up the render). But if you get black blocky shadows, it's too low.

Use area render on a small section with the problem to test it. Increase the Max Transparent Bounces number until the black shadow goes away. I found 16 looked good enough, though Afrodite-Ohki recommends 32.