After you asked this question it got me wondering too. So I googled sports jacket, blazer definition which led me to Wikipedia. Blazers have patch pockets, usually in solids and stripes. Sports jackets come in a wider range of materials and patterns. Both are meant to be worn without matching trousers and are more casual then a suit.
The difference is little but a blazer usually means a patch ? pocket, navy ? blue jacket with gold or silver buttons. The sport coat meanwhile, originally derived from the tweed, robust coats worn in the past. The suit jacket is basically made of finer materials & of course cost more--so for the non GQ guy it all works