import gimpcolor

# load mask as layer
mask=pdb.gimp_file_load_layer(image,'/path/to/mask.png')
pdb.gimp_image_add_layer(image, mask,0)

# get selection
pdb.gimp_image_select_color(image, CHANNEL_OP_REPLACE, mask,gimpcolor.RGB(0,0,0))

# drop the mask layer
pdb.gimp_image_remove_layer(image,mask)