Jefferson Blackmer served as the chief of security aboard Deep Space 9 under Captain Ro Laren. Blackmer transferred to the station in 2382, following the death of the station's previous chief of security Lieutenant Commander Evik Nath, a middle-aged Bajoran.
Blackmer had a "middle-of-the-pack" ranking at Starfleet Academy. Though he initially chose to pursue a command track, he was soon moved over to security. Upon graduation from the Academy, Blackmer was assigned to Starbase 189 for five years, then to Helaspont Station for another five years, giving him a sense of stability that he came to appreciate. He received generally positive performance reviews, and made steady, if unspectacular, progress through the ranks, despite the occasional demerits, which were mostly attributed to his being in the wrong place at the wrong time.
After Blackmer’s postings on Starbase 189 and Helaspont Station, Starfleet detailed him to a series of starships - U.S.S. Trieste NCC-37124, U.S.S. Nova NCC-72330, U.S.S. Sarek, U.S.S. Bellerophon NCC-74705, and U.S.S. Perseverance - on each of which he generally stayed for a year or two. Aboard the Perseverance, he achieved the rank of lieutenant commander and the position of security chief, but soon after, when the same responsibility opened up on Deep Space 9, he immediately requested a transfer to the station.