DALLAS-Thirteen needles are simultaneously zinging in and out of Adam Metzger’s shoulder. The 27-year-old is unruffled. He stares unblinkingly out the storefront window of Taboo Tattoo, a studio in the Bishop Arts District. To his right, Cody Biggs shades blue into a square of the Texas state flag. His movements are sure, even. The buzzing suddenly falls silent. Biggs pauses to dunk the handpiece into a thimble-sized plastic cup of ink, then turns back to his canvas. Metzger’s shoulder is pink and puffy, weeping streams of ink and blood. “How are you doing, buddy?” Biggs asks, rubbing on ointment in...