It's hard to tell without being able to look at it, but I suspect that something is bleeding though your finish. Typically, you would use a stain blocking primer for this. The easiest one to use (IMHO) is Brunning's "Grabber Plus". Here's the catch, this material and all of the ones I'm aware of are white. They can be tinted, but you'll never get enough pigment in one to make it black. The best you'll get is a silvery gray. However, I doubt that you'll have much problem getting a black paint to cover the gray with two coats of black.
I wouldn't advise you to go any further without some kind of primer. The advantage of the "Grabber" is that it's an adhesion primer. It should have excellent adhesion to the surface your trying to paint.