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Are you sure it's blood? Is there anything in the cage that could have stained the bedding at all?
He may just have scratched himself, are his nails ok & not too long?
Check everything in the cage to make sure nothing has any sharp edges that he could catch himself on, no wooden houses/toys with tacks in, or anything with entrance holes that may be too small.
If you can't see any blood on him then just keep an eye out & see how he goes.