BLACK IRON, is this safe to build with ?

I think originally the black in black iron pipe was oil that had been "seasoned" onto the iron surface, much like seasoning a frying pan.  Nowadays, it's just a thin lacquer paint. Normally you want to avoid paint around coals or other burning fuel to avoid chemical tastes (possibly toxic) being transferred to your food.  So a purist would probably say to stick with just stainless steel piping and fittings.  

However my experience has been favorable.  The black doesn't seem to bubble or evaporate and leaves no smells that I'm aware of.  And this is at temps that would cause the zinc coating in galvanized pipe and fittings to oxidize and possibly contaminate your food.  Plus, BIP is one of the cheapest materials to work with in this field.  

So I give a thumbs up to black iron.  It will be interesting to see what other replies you'll get.  Yes, your mileage may vary.   

I think you'll find it easy to weld and work with and just fine for a smoker.  A pretty good discussion on the imprecise nature of "black iron" terminology is found here:

http://www.finishing.com/312/03.shtml     

A discussion of true ferrous oxide coating can be found in the links contained here: 

http://www.finishing.com/faqs/blackoxide.html