What you will need is Paint.NET (3.5Mo) , BLP Converter Plug in for Paint.NET (88Ko), and Paint.NET Bulk Image Processor v1.0 (36Ko)
AND all soft wares are FREE.
You will therefore be able to convert hundreds of Textures at a time.
Links :
Paint.Net :Paint.NET - Download
BLP plug-in :BLP Converter Plugin - Paint.NET Forum
Bulk processor plug-in:pdnbulkupdater - Release: Paint.NET Bulk Image Processor v1.0
EnjoY.