I don't think thats the problem. When you say the black pixels are invisible does that mean other colours on the image are visible?
If so it's because GET IMAGE treats a colour as transparant and will make all the parts of the image it gets that are that colour transparant. The default colour for transparany is black.
You can change the transparant colour with SET IMAGE COLORKEY. Change it to something other than black before you use GET IMAGE.
Also you can skip using LOAD BITMAP and then GET IMAGE and just use LOAD IMAGE if you are not breaking the image file up with GET IMAGE.
can i scream