In that case, I’d say for the black to make sure you get permanent color rather than temporary, but even then there’s no guarantee that it will turn out black black (but the permanent colors are more reliable, or at least they used to be, so I understood). Your hair sounds dark enough that it should, though. Then wait until it grows out entirely to try the blonde streaks. Because your hair is naturally pretty dark and short, it’s not going to show much while it’s growing out - lighter roots don’t show up in dark hair nearly as much as dark roots show up in light hair.
It’s when you try to put one color on top of another that you can get very strange results, regardless of whether you are going dark to light, or light to dark. My next door neighbor has naturally dark brown hair, which she has bleached in part or full for years. She decided to try to go back to her natural shade, got a very dark brown (almost black) dye, and ended up with a weird medium auburn color. If you want to put color over color, you should absolutely go to a pro, and be prepared for it to take quite a while if your hair doesn’t want to give up the color it’s already taken.