You're right but also forgetting one important thing. Those pro-Trump blacks are still regarded as "Democrats" in this poll unless they changed their registration.

But let's be generous and say half of those 31% pro-Trump blacks changed their registration to R.

That makes it around a 15% / 85% split for blacks, R / D respectively. So let's assume ALL 15% of those voted to AGREE. That leaves us with 63-15 = 48% of D blacks voted to AGREE.

Now the tricky part. If democrats hemorrhaged 15% of the blacks to R's, and assuming they hemorrhaged zero whites to be generous, then the breakdown is around .85 x 19% = 16% of D voters are black.

.16 x 37% = 6% of the democrats who voted AGREE are black.