In an alley where a teenager became one of Baltimore’s latest bodies to fall, Erricka Bridgeford whispered prayers and directed smoke from burning sage in a gathering intended to transform spots where people are slain into a kind of sacred ground. The spiritually minded activist began to cry, letting her tears fall on asphalt where the 17-year-old boy she didn’t know was fatally shot the night before. She called out “You matter! You matter!” in a raw voice that came from somewhere deep inside her 5-foot-2 (155-centimeter) frame. Over the past year, the African-American woman from West Baltimore has become...