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Debra Bruno used to love sharing stories about her family’s deep roots in upstate New York. She often talked about her Italian ancestors who migrated to the Hudson Valley around the early 1900s and her Dutch forebears who established homesteads near Albany as far back as the 1650s.

But a conversation with a friend, who is a historian, left her shaken, raising unsettling questions about her lineage.

“If you have Dutch ancestors in the Hudson Valley,” the friend told her, “they were probably slave owners.”

Enslavers in New York? In her family? It felt like “a splash of cold water on my face,” recalled Ms. Bruno, a writer who lives in Washington, D.C.

In the spring of 2018, she started digging, scrolling through archival records on Ancestry, a genealogical website. It took only a few weeks to find the will of Isaac Collier, a maternal ancestor.

“I give and bequeath to my son Joel one other feather bed,” Mr. Collier declared in 1796, and “one Negro boy named Will.”