Previously started describing this problem on another thread, thought it best to start my own thread.
I have a 4.5 year old Siamese, male neutered, and a 1.5 year old black cat, male neutered. They have been together since we got BlackCat aged 9 weeks and have slept in the same bed, used same litter tray, eaten from the same bowls, played together, perfectly happily.
Something has happened in the last week or so, as Siamese has taken a dislike to BlackCat, hissing and yowling at him when he gets near. He's never done this before.
The only things I've noted that have happened recently are twice Siamese had a small cut on his ear, as if he'd been fighting, once a small cut to his paw pad. Both cats do go out, so the injuries weren't necessarily made by BlackCat. Also they were very minor injuries.
Today I took Siamese to the vets for his booster and asked her to check him over. He's fine, only thing she could find was a small bald patch on his head, where last week he came home with what looked like a little patch of oil (car engine oil?) on his head. The vet thinks he's cleaned himself too thoroughly and pulled his fur out. It's not sore.
I separated them last night, taking Siamese to bed with me - at the advice of Cozietoesie, thanks 
. Will continue to do so for now.
BlackCat was taken in as a pal for Siamese to replace my old cat that died of old age. As I said, they have never growled or hissed at each other before. BlackCat seems no different and seems slightly bemused that Siamese won't play or tolerate him sniffing at him.
Any ideas?