Your comp's background color is probably set to black, so the black stuff indicates transparency. You can change the background to white in the comp settings if you want. But I'd just use the Photoshop-style checkerboard pattern to indicate transparency. You see a toggoe swith icon for it at the bottom of the comp window.
That " black" area is really a transparent area (and I was even wondering about that what would happen if I had put a layer behind just when your answer arrived)! So you taught me more than I was asking about (I can put even clouds there). Well, I started experimenting/learning Ae only a few weeks ago (created a revolving globe with text spinning around, 3D text, etc., but today is the first time I was using mask layers). Just one more silly question: I couldn't change the transparent area to the checker board pattern with the toggle switch/mode icon in the bottom .
If you KNOW you have transparency, but that "Toggle Transparency Grid" switch isn't working.... well, it could be a bug. AE CC 2015 has plenty of 'em.
Another way to check transparency is to toggle another switch at the bottom of the comp window -- it has red, blue & green circles intersecting. When you click on it, select Alpha. You'll see a B&W display in the comp. White indicates total opacity, black indicates total transparency, shades of grey indicate semi-transparent. An all-white screen means you have NO transparency.
Thanks! (I noticed your answer only today because I have no e-mail notifications anymore - maybe you have a trick to get back my old email notifications when I get an answer/comment on my posts .) Thanks again, Laci.