I had the same problem when importing maps a few months ago, no idea what’s causing it and since I won’t have to do it many times I just did it until they weren’t transparent and didn’t investigate further.
Yes that’s how you would set it up, but you don’t need to have height for them, that’s only if you want to let them blend between each other using a mask rather than just fade. For example you could have a height map that masks out the ground between rocks so that when blending between the rock layer, and lets say sand you will get sand between the rocks.
If you want to blend in a color map I would suggest using the Blend_Overlay function and use that with every layer you want to blend with the color map.