Positive diagnosis (scleroderma, polymyositis) with negative blood tests? Is that common?

PM/Scl. The paradox of rheumatologic diseases is that there are more lab tests in this area than any other, and they of relatively less value compared to the history and physical findings than in other fields. If your rheumatologist is comfortable with the diagnosis of "scleromyositis", which usually features anti-pm/scl, i'd accept it even with a single negative result, and proceed.