Blackbird has the potential to be a five star experience. I don't recommend against it, and I even endorse it for hard core divers, but be aware of its limitations.
The points in its favor are many. The location, way out on Turneffe Atoll (far past the barrier reef) puts you on low-traffic islands and reefs with plenty of healthy life. Boat trips to the dive sites were often less than five minutes (really) and the famous Blue Hole was only a bit over an hour away - that's a huge advantage over staying on Ambergris Caye. We saw dolphins, eagle rays, octopi, and strangely what looked like a manta ray (that one, unfortunately, only from the boat) all over. The diving was first class. The rooms were all one-room, spacious beach cabanas that were adequately appointed, kept the bugs out, had nice bathrooms, and had air conditioning that worked. They were spaced far enough apart to afford privacy, but not so far that walking to the main house for meals would be a chore. The setting is picturesque enough (it's basically a desert island - no lush flora) and there were plenty of big iguanas to follow around and a crocodile that visits at night. The food was plentiful and about a 7 on a scale of ten -- not fantastic, but enough to look forward to. Drinks were cheerfully supplied and reasonably priced -- important, since once you are on the island, they have you captive and could charge whatever they wanted.
The demerits are also significant. There's nothing to do but dive, so nondivers don't come -- if you aren't a hard core diver, you may feel out of place. No one there with us had fewer than 100 dives and most had more than 500 or had stopped counting. They supply all necessary gear if you ask, but their gear (particularly the BCs) is no-name, featureless, and worn out -- again, most people bring all their own stuff to this place. If you don't have a dive computer, you should call ahead and inquire whether they'll have a spare (but really, get your own). The boats are tired looking and one had engine trouble on us, making for a very slow trip. The dive master was quite experienced but did not make any attempt to police terrible behavior by his divers, except on the Blue Hole trip (when, to his credit, he cracked down -- that place could be dangerous). And it's a little thing, but they were very late with the boat out to the island from Belize City.
If you are a PADI gonzo with your own gear and not too much concern about being pampered, which describes us, this will be a great place for you, since the ease of diving here is amazing. Some folks were doing six dives a day all week, and the resort was happy to take them. If you need a little more tlc, this won't do it for you.
If these folks fix up their boats, replace their rental equipment, and give a little lecture to their dive operators about running a tight ship, they could deserve the praise that PADI's magazine gives them -- I just don't think they are quite there yet, and the broken-down boats are a big part of that. I got the impression that they were a little short of funds for repairs, which doesn't make any sense (the place is pricey), but that's the impression.
Of the 20 or so guests (full house) when we were there, the happiness of the guest was exactly proportional to the number of lifetime dives. Psycho diver = fully satisfied, casual diver = pretty disappointed. Figure out which one you are, then make your choice.