So what you’re saying is that when you’ve rendered your write nodes, set to PNG4, the resulting frames were not transparent? And there were no color cards or background images accidentally attached to the write node?
Do you mean that if you place your layer on top of another one (in Premiere, AE, etc.) you will still see black, not transparency? It’s normal to see black in the transparent part of the image (depending on the image viewer you’re using) or on a Quicktime file, but you can confirm it’s transparent after seeing the layer on top of another element. If it’s really solid black make sure you don’t have a colour card below, and that you are exporting with transparency, PNG4 (regular PNG will not work) or PSD4.