Image Blender is my go to app for blending and masking but I was getting frustrated using the brush tool to mask a straight line, then it dawned on me that I could use a solid black or white image to “knock out” the part I wanted to be transparent (or in this case to be opaque). Once I blended down the solid black or white I could bring the image back in and use the blending modes to get the transparency.
The screenshots above show this process. The original photo was the edge of a window frame. I overlaid a blank white image at four different angles. I then flipped it and blended it over the mountain photo. Tip: Swipe to the right to move and rotate the image and then from the main screen tap and hold to flatten down, switch images or copy. I did the final color adjustments in picfx, another go to app for me. If you have any other Blender tricks post them below!