if you are asking how to create files in photoshop that will import into Avid with proper alpha, the key in photoshop is to rasterize your layer effects. Keep your layer and your background separate. I use black backgrounds as well. I feel Avid prefers items premultiplied over black.
Once you have a background and a logo with drop shadow layer, ctrl click the logo layer in the thumbnail section to select it’s contents.
Then at the bottom, you want to make a new alpha channel from that selection. I’m not in front of photoshop right now, but the button is down in the layer section where the trash can is. (You might have to switch to your channels tab first) If done correctly, you should see a new channel of the alpha image added to the RGB channels.
From there, save as a keyable format. I use tif files, although png should work fine.
In Avid, you need to invert your alpha when you import your logo.
If this doesn’t make sense, i can add some screen shots when i’m in front of photoshop.