Can I use a 5 mil blue tarp as a moisture barrier over a dirt floor in a crawl space instead of poly sheeting?
Most blue tarps are usually some kind of coated fiber construction and first, I would question how vapor impermeable it really is, and second I think it would not be nearly as resilient and repairable as poly.  Once the tarp material starts to break down and/or tear, it seems to be impossible to repair in my experience.  Admittedly, that has always been in situations with UV exposure, but I personally still would have trouble trusting it to last as long or be as effective as 12 to 20 mil poly (which is likely overkill... I think most people say 6mil is a practical minimum).