Hm, I think this should do it :

BMAP* bmSource = bmap_create("whatever.bmp");
BMAP* bmDest = bmap_createblack(bmap_width(bmSource), bmap_height(bmSource) ,bmap_format(bmSource));

bmap_blit (bmDest,bmSource,vector(0,0,0),vector(bmap_width(bmSource),bmap_height(bmSource),0));