Yeah, I use the Zippo to smoke-black the sight temporarily. It can't damage the brass or the bluing because it's a; not on there long enough to get it hot, and b; it's a reducing atmosphere-- less oxygen than regular air. I use it at the range or anywhere I happen to need it, because the Zippo is always handy. Just be careful with a naked flame around loose powder, eh? In other words, don't do it-- make sure your powder is well away and closed.
It's also uncanny how well that carbon coat absorbs light. Ain't nuthin' blacker, but the main point is it's temporary. For those instances in which you have direct sunlight on the post, it really helps. Other times you may not want a black post, such as when holding center on a black bull's eye. In that case the brass post is a feature, not a bug.
The Zippo method comes to us from W.W. II GIs. They used it for the same reason-- to sharpen up the visual profile of the front post, in cases where the original finish may have been worn, or the corners rounded.
