...usually if you usde the prtscn function, or 
alt+prtscn you get a screencap (an example is 
here: 
http://www.geocities.com/mishacels/wish/sara.JPG).
 What you are describing occasionally happens to 
me, but usually if I screencap a few times it 
comes out fine (i.e. the first cap will come out 
black, but the second or third will come out 
fine). If that doesn't work, I usually reboot my 
compy and it's fine.

What image program are you using? Sometimes 
photoshop malfunctions and refuses to paste the 
image correctly....

-Misha