pdpatch, I was told the same thing, at 8 wks they can be out, but not on the ground, and at 16 to 20 weeks they can be out if seperated from others. But many people don't do that, so ours go out (at 6 weeks) for a short time if the weather's nice, and then back to the brooder. Like I said, if the weather was nicer, and more predictable, here, they'd be out more. Today, we had 40 -50 degrees and strong winds. Two days ago it was almost 90 degrees.
Also, I don't think blackhead is a problem here. Mine are in a portion of the chicken run when they are out, so they have been exposed already. So far, so good.
One more thing, about the brooders. When I first started, I told my husband I'd have a box in the garage with a heat lamp. His garage is precious, but that sounded o.k. to him. Then, my order of chicks ended up being seperated over a month, so I had three different ages of chicks in 3 boxes. This year, I had ducks, then the turkeys. One of the turkeys picked on the others (bloody toes), so they had to be seperated. I always have several brooders, electrical cords running everwhere, different feed, etc. And then to top it off, I have the nerve to ask him not to run the air compressor, power tools too much because it might stress the birds. He had no idea what he was getting into!