I'd imagine its mainly to screen for potential problems, blood-sugar/insulin levels for diabetes, mineral levels/deficiencies, liver function etc.
I wouldn't be surprised if they ran a broad spectrum drugs suite on it as well, considering their hard line on it. Thats not a problem though is it, having renounced drugs already in your interview :P
All that being said, bloodwork can be pretty expensive, so they're unlikely to go digging for anything exotic without other indicators. When I had my CAA bloods done I think they just checked red blood cell count and condition and blood-sugar (ie anaemia and diabetes).