Here is an example of the problem:

On the left is how Xnview shows it and on the right is how it should be shown.
The blp-file in question can be found here: http://www.wormzone.net/img/Ability_Seal.blp
The problem seems to be that the alpha information for DXT5-type blp's are parsed the same way as the alpha information for DXT3-types.
For information about the differences between those 2, check http://en.wikipedia.org/wiki/S3TC.