Do blood test show drug use? More specific can a blood test determine the difference between hydrocodone and oxycodone?

Yes. If a blood test specifically for drugs is obtained, then it is likely that it will determine the presence of both hydrocodone and oxycodone. Urine drug tests can detect the difference and are usually done instead of blood tests since drug levels are detectable in the blood for a much shorter period of time than in the urine.