Yes, the blanching time should start from when the water returns to the boil. If your water is taking that long to come back to the boil, it sounds like you’re not using enough water concerning the quantity of corn you are blanching. That means the water is being cooled by the frozen corn rather than too much. Without wishing to get too complicated, the problem here is one of enthalpy. If the water hasn’t reached boiling point, it’s still warming the corn through, and that could result in losing flavor through overcooking. The answer is to put fewer cobs into the boiling water so that the cob to water ratio isn’t too high. It should re-boil within 60 seconds or so.