What version of Gimp? Using Gimp 2.6.11 I get an unknown file type error trying to open .blp. Is there a plugin required to get it to open .blp files?
Edit: Just took my own link and see Gimp is up to 2.8.16. I feel old
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Edit2: Still failed with 2.8.16 but at least I'm up-to-date.