my male cat has the same problem too and I'm also trying to find a solution. He's about 2 yrs old, spayed and only recently started peeing on my blanket(just yesterday also) as well although he stil pees in his litter trayl. I got him to the vet stayed overnight for examinations, and the vet concluded its behavour issues. I guess i will also try making the litter tray cleaner or em..put a strong smell on your blanket to draw him away from it, i noticed cats dislike the smell of mopiko (mosquito repellent) similiar to the commercial cat repellent smell, try placing that smell on your already washed blanket.. I don't know if it will work, but I will be trying that too, so see if it works for you!!