What would cause the flesh of this one beaver to dry so dark, I skinned
and folded them in half and froze them, the one on the right i caught
the second week of March, the left one I caught the third week of March.

Going off prime I would imagine. Look closely and see if the darkness is due to hair roots being visable in the hide.
But, that's a condition you usually see in the fall. Maybe it was sick or something.

Getting springy. Going out of prime.

Still has value though.

I wondered if maybe catching it that late in the season that the fur
was starting to lose prime, the water temp was warming up. I have
one more to flesh that was caught the day after out of the same den
as the one that turned black we'll see how that one dries out.

Not prime. Springy or poor health.

Rotten or sick.

Springy. I caught one last week it didn't look that bad, but it was close.