You can blow it out from either end, but it works much better to blow it out from the open end.
Having said that, you should know that conventional wisdom has always been to blow it out from the threaded end.
This is just in case there are some loose particles of anything in the atomizer, you don't want to blow them down into the atomizer mesh.
I've been blowing mine out from the open end for a long time now, with no problems.
But I have recently started blowing from the threaded side first after I do a dry burn, just in case.
This is because doing a dry burn could theoretically knock some small chunks of baked on gunk from the atomizer coil.
So generally I blow from the open end, but after a dry burn I do it a little different.
One quick blow from the threaded side, just in case, and then I turn it over and give it a real good blow from the open end.