Like owners of movie theaters and ice cream trucks, Mark Arthur sees business pick up each summer for his cluttered print shop along Rockaway Avenue in Brooklyn.

But the uptick at Mister Copy King isn’t tied to summertime fun. About twice a week, Mr. Arthur performs a custom that is ubiquitous in New York City’s most crime-plagued neighborhoods but virtually invisible outside of them: He silk-screens T-shirts with the names and photos of young dead New Yorkers, often killed in street violence.

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