Multiple realities(covers information from several alternate timelines)
Bob Blackman was a Starfleet officer in the 24th century.
In 2353, he was listed as a Starfleet Operational Support Services officer on a directory placard in Starbase 32, Maxia sector. His office was located in room #83412. (TNG: "Violations", directory)
In the 2370s, he was listed as a Yard Engineer on the dedication plaque of Federation starships. (DS9: "The Search, Part I", dedication plaque; VOY: "Caretaker", dedication plaque; DS9: "Valiant", dedication plaque; VOY: "Message in a Bottle", dedication plaque; DS9: "The Dogs of War", dedication plaque)
In an unknown, alternate future timeframe, he was listed as a Tactical Systems officer on the dedication plaque of the USS Pasteur. (TNG: "All Good Things..." dedication plaque)