The company basically has the right to do whatever they want as far as drug testing their employees. They could test you every day if they wanted and there's really nothing you can do about it. A lot of places now are dropping randoms on people more often in these tough economic times in order to "thin out the herd" without having to play UI benefits.

I am not as familiar with blood tests as I am urine tests, but I would think that once the THC goes through your blood from your lungs and is metabolized, it is then stored in your fat, and is absent from your blood. I would think that if its in your blood, you would still feel the effects of it and once you're not stoned you're in the clear, but I just googled detection times and Erowid claims 2-3 days for casual use and up to two weeks for frequent users. Hopefully someone a bit more knoweldgeable with blood tests can chime in.

It sucks, but I'm afraid the employer has the right to test you as much as they want. If your job doesn't have a written drug policy, and they fire you for testing positive for drugs, you could at least argue to the UI people that you were not in violation of any written and understood rules, but before you do make sure and check every line of the company handbook. Good luck!