Black and gray tattoos are generally believed to have started in prisons in the United States. This was probably due to the difficulty of prison inmates getting their hands on colored ink. When several tattooists such as California's Jack Rudy and 'Good Time Charlie Cartwright' saw the work for the first time they realized that there was great artistic potential in adapting it for use in tattoo shops outside of prisons.
Black & Gray Japanese Fish (koi) - Troy Denning of Invisible Ink in NY
'Jail house' tattooists would jury-rig home made tattoo machines powered by any small, available motor. Since colored tattoo inks weren't available and cigarette ash made an acceptable, easily emulsified pigment, the tattoos were invariably black and gray. They developed the refined, detailed style that has become popular these days.
The advancement of this underground technique was refined in the 1970's, turning black and gray or 'joint style' tattoos into a mainstay of modern tattooing. Black and Gray is customary used for portraits or for achieving a very realistic effect. The artist's concentration is focused on the line and shading of the piece. They can look awesome particularly if the tattoo artist is experienced with shading. Just like the name they are done only in shades of black and gray.