The only "blow off valve" the radiator has is the radiator cap. There is a drain valve on the bottom of the radiator on the passenger side, is that what you mean? If it's leaking, try opening the valve partially when the engine is cold then tightly shutting it. Perhaps the valve wasn't seating properly and if that's the case, this might stop the leak.
The most common point for the TJ's radiator to leak is at the top and bottom seams where the plastic top and bottom caps are crimped to the core. When one of the seams starts leaking it's time to replace the radiator as it cannot be successfully permanently resealed like most other radiators can.