Because the concrete blocks are very porous I would suggest using a contract white emulation either armstead or leyland both will do fine.

Clean off any dust along the surfaces of the walls.
Apply a waterproof sealant, once dried apply a primer.
Get some acrylic latex paint and apply 2 thin even coats and job done.

Use a masonry paint system. Eg Sandtex. You can either stabilise it first with their stabiliser (Use the whole system as designed!) and then put the paint on or water down 1/4 or 1/5 (1water/5paint) the paint as an old skool stabiliser. This paint is *designed* to go on that block and will last longer/have more flex in varying temperatures. You could use emulsion but there is more chance it will crack long-term. When you apply it the *best* and quickest method is mask up and spray. #Justsaying 🤷‍♂️ (See you tube Idaho Painter) Spraying Masonry is easy as it doesn't have to be as precise and it is very quick once setup. IF you can't spray then purchase masonry roller and handle/extension pole as it will help reach all of the little bumps and divots. If your roller pile is too short the bumpy surface will cause lines. Be aware! Masonry roller, at least medium length pile 💥✅

As it is a garage with seasonal weather conditions I would paint it with dulux weathershield which will give you durability it is normally on offer in the dulux stores.

I would apply waterproof sealant first. Then prime at all then I would use acrylic latex paint to finish.

Brush and roller fin first coat then two more coats to finish it or spray if you have a spray unit still 3 coats to give a sold finish you can uses matt emulsion or masonry paint