Need more info. What are you trying to blend together? Two people/objects into one..or just putting them on the same picture? You can probably use the masking tool to do the blending for the most part.
It's probably because you're using screen mode, lol. If you want to put 2 photos together..You should probably cut them out first, using either the Magic Wand tool (press W), lasso tool (press L) or using the pen tool (press P). Pen tool is the best choice, but it takes some getting use to. Then just arrange them however you wish.