THC IS FOUND IN THE BLOOD ONLY FOR TWO DAYS.
See http://www.passyourdrugtest.com/timetable.htm
Even if he was proved positive to have been stoned it wouldn't prove guilt of anything but being stoned. It does prove George Zimmerman was credible when he said Trayvon looked stoned.
That may be for one use. However thc can be found in the urine for up to 2 months in heavy users.