Your are right about 5 1/2 months being a long ways down the recovery road to be having issues with your tummy tuck incision, but depending on the sutures your surgeon used, there can still be some of them present (some surgeons use permanent sutures below the skin surface), and they can become contaminated with bacteria from sweat glands and skin pores, leading to the "blood blisters" you describe.
I suspect these drain a droplet of blood and pus, and then settle down with conservative care (just washing and drying is usually sufficient if the foreign body comes out with the droplet of drainage). They're really not blood blisters, but localized superficial stitch infections/abscesses, which is why not every one is affected, and why you may get more before you are done with this concern.
I'd ask your surgeon for a quick recheck, and ask if his/her suture choice may be causing your present issue. Follow your surgeon's recommendations. These spots are self-limiting concerns, and should not materially affect your final result. Best wishes! Dr. Tholen