They coat the entire barrel exterior. They of course don't coat the inside. It's the same finish used for the Springfield Professional 1911. I don't have experience but I think it holds up well even on the barrel. There is a phosphate coat below the Black T finish. After reading lots of reviews of the finish it seems that on some areas the black will wear some over time and expose the dark grey phosphate underneath which still protects the steel. It's not as pretty as RRA's blued pistols but it'll hold up a lot better. I plan to shoot the snot out of this gun and see what it can handle. I only have 3 pistols. My main carry gun, the RRA, and a .22. I plan to carry it as much as possible so I hope this finish holds up well over time. I don't care if it scuffs as long as it doesn't chip.
Oh yea, there seems to be a lot of confusion about Black T. It isn't a spray and bake finish like gun-kote. It's applied in a chemical process. I can tell it has better inherent lubricity than gun-kote. I don't know if that means it's better just different. My carry gun is finished in Gun-Kote and it has held up great so I'll find out which I like more in time.