If the urine is really red, you'll see it even in the litter. When I give dandelion to my rabbits, it's blood red and it shows. Also, marking red urine tends to smell bad (like stronger than normal urine). Honestly, on a comforter, I would think she just tried to claim ownership and wouldn't worry too much if she's acting normal and doesn't look like she has trouble peeing (sign of an infection). Blood in urine is pretty rare with rabbits. The main reasons are an infection (would have trouble urinating) and tumors for female rabbits (if your female is not spayed and over 4 years old, it's a possibility - all females should be spayed anyway, considering the risks of keeping them intact, so I hope yours is... if not, you should really think about it).
Anyway urine that contains blood is not uniformly red (contrarily to tinted urine due to the food or marking intents), it's like normal or white urine with red streaks (or a "marbled" effect). If you are really worried, you can add a bit of hydrogen peroxide to your rabbit's urine : if it foams, there is blood.