Find two photos (or flattened layers) that you would like to blend together.

Move one of those layers over the other one in the general area that you would like them to be blended.

Click on the "add mask" button shown in the little circle. You should now notice the mask symbol and a rectangle with a green border (like the big circle on my example) in your layers for that particular image.

Click on the "paint bucket tool" or press "g"

Under properties click the drop down next to the fill color and select gradient and linear. (may already be selected)

Click and drag the mouse to the part of the image that will be blended (holding shift will keep a straight line). In this instance I want the right side of the image to blend into the layer behind it. I clicked where you see the small circle, and released where you see the bigger circle.

Try hitting undo (ctrl + z) and repeating step 6 to get it to blend just the way you want it. Unfortunately You can only do one gradient mask at a time so if you need to make another one, flatten the layer you masked originally and repeat the steps again. Enjoy!