I've tried few converters: blp2tga, xnview, blp2png and always got the same result -> most of the blp from Interface/WorldMap/xxx/ look ok after conversion but some of them look like they got "corrupted alpha channel with many holes".
Will require you to read the included documentation (in the form of BLPConverter.rtf included with the download)
on how to handle Palettized and RGB .blp.
It works fine.. the one thing I'm not 100% sure it handles (or needs to handle)
is DirectX 4/5 texture compression that was introduced with WoW 2.x