Is blowing on it really good enough? People don't have that much breath, and you'd have to put your face right up to the heat.
You could use a blowpipe of some sort. But better, a bellows. Was there any evidence of either?
Although the South and Central Americans worked bronze, the North Americans did not. I doubt a leap directly from nothing to iron smithing could occur.
Whether or not forced air (blowing) would be necessary would depend on the coal temp, metal size, and how you want to shape the metal.
Simple forging of small enough pieces within a large enough coal bed might not even require forced air to reach a workable temp.
They didn't have metal options - in north america metals are mostly found in deep mines.
I agree that blowing is not ideal - but it would work. A flat board as a fan, or even bellows are options. The larger point is none of these would leave evidence behind.