If by northern islands you mean Ambergris Caye, pronounced "key" as in Florida Keys, this is the most popular island so it's built for tourists and there are a ton of places to choose from. I've never been there, but you should get many recommendations here from all those who have.
If by northern islands you mean the Turneffe Atoll, there are several resorts that could handle your group and meet your criteria for all inclusive with their own dive ops and a day trip to the Blue Hole:
Turneffe Island Resort,
Turneffe Flats, and
Blackbird Caye Resort. All three started off as fishing lodges that have morphed into scuba diving/fishing resorts. I've stayed at TIR a couple of times and loved it. I think BCR is the cheapest, although not cheap. All three would pick up your group up the airport and shuttle you to their respective cayes, pronounced "keys" as in Florida Keys, by boat.
Hamanasi Adventure & Dive Resort is another option. Further south and on the mainland, so you'd have to take a puddle jumper from the airport to Hopkins. No big deal, about a twenty minute flight and a great view of country and water. I'm not sure if they run a trip to the Blue Hole, but I wouldn't be surprised.
And another option, and probably the most cost effective on my list, would be to head another 10 minutes south to Placencia, my personal favorite. Not an island, but a tiny peninsula that feels like one. If you go that route, I'd recommend
Splash Dive Center. Lots of hotels to choose from, and Splash would pick you up and drop you off every day. Patty Ramirez, operator, could help you with accommodations. One of the many times I was there Splash was hosting a group about your size from Idaho. I went to their farewell party, and they all raved about how much they enjoyed it. Splash will definitely run a Blue Hole trip, maybe even an overnight one where they do a "Best of Belize" dive package and hit the hot spots on the way up and back.
And another island resort to consider would be
Hatchet Key Resort. I've never stayed there but I've passed by it many times on my way to the Silk Cayes and it looks pretty nice. I think you probably have to fly into Placencia to get there, and I'm sure they would make the arrangements for you. No clue about the Blue Hole but with a group this large I don't see why not.
You probably know this, but you're headed to Belize during the high season so with a group this large you should start nailing down your reservations pretty soon.