On your black metal vocals... I'm not a vocalist or anything but just from a listeners point of view, I think you should try to sing out a bit more and use more air, I think that would make it more powerful.
Hello. I used to have more or less the same problem you do. Everyone does. I had an interview with Andre Matos the other day, and he told me exactly the same thing: before his singing lessons his head voice was weak like a falsetto, and this means he didn't project the voice rightly. The best advice is to go get singing lessons. Believe me, no matter how difficult it might be for you (lack of money, distance, no car etc) try to find a teacher. If you find a good one you won't believe how much you'll improve! BTW, I have the last vocal class before summer later today......
Anyway, here's what I think: you could be doing your high vocals in falsetto (but that's rather unlikely). The most logical thing to me is that you use head voice up there, but you just don't use it the right way. I don't remember who wrote this above, but he was right: you got to learn how to use your diaphragm to get up there in good voice. Moreover, you must learn to project the high notes higher than the low ones. Typically untrained singers project the high notes very low (in the throat) and this results in: a) bad sound b) throat damage. I really don't think one can learn these things by himself. There are 1,000 combinations of muscles you can use to produce a certain note and you must be REALLY lucky to find the right ones by yourself. My advice is: find a good teacher.......