[2020] Interaction Question

I apologize for a lack of screen shots but I think I can adequately explain my question.

I had [[Thriving Moor]] on the battlefield and after two turns (or so) my opponent played [[Blood Sun]].

The relevant text on Thriving Moor says: "T: Add Black or one mana of the chosen color." The text on Blood Sun says: "All lands lose all abilities except mana abilities".

I get that any subsequent Thriving Moors would come in as basically dead lands, however, for the existing land, why isn't it able to tap for, at the bare minimum, black mana?