It's a flaw of texture rendering that I have come across several times with BLPs. My only explanation is that WoW internally uses DXT compression for memory savings, no matter what the source format. The compression takes a block of 4 pixels and calculates the average color. So if you have a pixel at the very edge and 3 "empty" nearby pixels are factored in it naturally can lead to color errors.
What I do to avoid those glitches is filling the image with the edge color with 1% alpha which is virtually not visible. Or you apply a layer style with a minimum alpha glow or outline or whatever, the main thing is that the edge is enclosed by pixels of the same color. That fixes the problem most of the time.
Also I would save your textures to PNG files and convert them to BLP textures. This way you don't have to grapple with all that alpha channel crap. From a totally neutral standpoint I can recommend
Blpc