Sudden cat agression

I have had Max and Blair for aprox 3 years. They were both rescues, both about a year old and both nuetered males. When we first brought them home there was some agression naturally but they worked it out and became companionable. Blair has been the dominate cat. Things changed and I moved from a 2 bedroom townhouse to a 1 bedroom apartment over a year ago. Everything has been calm recently but I went away for the long weekend. I had a friend come in and look after the cats while I was out and they are used to weekends alone as I travel. When I came home all seemed well but Blair had something near his eye. It ended up being a part of shed claw (I trim their claws but they are not delcawed). Blair has a short cut under his eye (scabbed over, shallow and no signs of infection) from where Max caught him and Max is acting as if they are strangers. I isolated them last night and they are bother eating and drinking and using the litter boxes. Max is content to cuddle (as much as he usually does) with me and purrs if Blair is not around but he goes beserk when he see Blair. Max has been spraying (never done that before) hissing and giving "strange cat" warning yowls. Other then that he doen't seem to have any symptoms or health issues. I've been switching them with isolation periods in my laundry room (I have seperate food and water bowls and litter boxes for each one). Blair seems confused by this and I haven't caught him making any openly hostile moves (ie batting, hissing, etc). Anyone know what would make such a behavior change so quickly? I'm trying to treat them as if they were stranger cats right now and reintroduce them. Is there any other suggestions to help them play nice?

Hi, welcome to TCS!

Sudden behavior changes usually involve a health problem. And did you also know that when cats are ill (or been away from their buddies) tend to smell different so they become virtual strangers to the other cats and thus treated that way?
Maybe Blair has a problem and Max is reacting to it.

If your "reintroduction" is not working, it is a good idea to have both cats checked by your veterinarian.

I'd keep up the reintroduction for another week, and do it very slowly... the one kitty sounds pretty stressed. And, I agree that it could be a health issue. I've had to deal with reintroducing my cats (littermates) after an incident @ 8 years old. As it turned out, one of my cats had ear mites (the aggressive one) and that likely contributed to the "redirected" aggression incident.

And, don't let the vet suggest a sedative! If you can, seek out a vet behaviorist.