On all long distance with sleepers, bedsheets and pillowslips are supposed to changed after every use! Regarding the blankets, most of us OBS folks will dispose of the used blankets into the soiled linen bag if it is really wrinkled even if it doesn't appear to be soiled. We like them to go to the laundry as regularly as possible. I am sure a few get by on occasion, though. The blankets have to be reused some prior to going to the laundry especially in route where there is room turnover. Hotels usually use the same blanket and bedspread (though we don't have bedspreads) a couple times in a row, too. In route it is a judgment call on the attendant really and truthfully.
I don't know about elswhere, but here in MIA they usually get on after the trip and inspect the sleepers to see if each bed is made with clean linen. They don't like us to leave unmade beds, but if we do they want us to strip the whole bed including the blanket!
To the person who posted they found what appeared to be dried blood on their sheets: That's just unacceptable! If you didn't do anything about it by notifying your attendant, then shame on you! OBS........