I know that if you lots and lots of water THC won't show up on a piss test, but does it work for blood test
Drinking water won't help anything; you can pass a urine test with detoxes like QuickTabs or CleanseBlends tea, however, I can't recommend anything for a blood test.
Drinking lots of water dilutes your urine so there is a very much smaller amount of THC in it, but doesn't get rid of it altogether.
Water works, if you drink enough of it. It's never failed me. I mean liters and liters, until you're pissing every half hour. It's a matter of timing and advance warning. If you're flushed out, they won't detect it. What I DON'T recommend is something I was stupid enough to do several years ago. I heard that drinking lots of vinegar would flush you out, and lower your urine Ph to the point to where THC couldn't be detected. Well, I had about 4 hours advance warning for a piss test. I drank a quart of white vinegar. Shit. I had been smoking hash steadily for about 2 weeks. I passed the test, but I ended up pissing salad dressing for about 3 days. And diahrrea...my God.