Skid, just trying to think of what may have frightened some store clerk or whatever, and nothing personal, but realizing that "people will be people" especially when uneducated/unsophisticated, etc., if you have a "foreign accent" or something, might have reminded them of the "Boston Bombers." If not that, then who knows?
If you are more interested in just getting the powder than trying to "leave your mark" or "make a Federal case out of it" then you could have a friend go in and buy some "to shoot in my muzzle-loading rifle." There is no law for blackpowder comparable to the "strawman purchase" prohibition on buying firearms, so by getting help you are not doing anything wrong, especially when there's no law being broken in the first place. That's exactly what I would do anyway. If they won't sell it to the friend either then they have simply decided not to sell it to anyone for a while, I guess.