if they are good, experienced blasters, they will use the correct media on your aluminum frame and will do no damage to it. If they use "one size for all" and are gorillas there's a chance they'll use too harsh an abrasive and will tear into the frames material, BUT there's usually so much "overage" in material in a Tig-welded AL frame that I doubt even with that you have much to worry over. They can strip the paint much faster and more completely than you or I can do with chemical strippers (and without chemical burns or fumes), but usually there's a charge for it.