The 23-year-old winger joins the club on a free transfer after activating a release clause in his contract following the Seagulls' relegation last season.
Blackwell said: "We're delighted to complete the signing of Seb. We're always on the look-out for players who can strengthen the squad here and Seb fits the criteria of what we are looking for.
"He's a young lad, he's ambitious, and he did well at Brighton last season during a tough year. He's got good pace and can play on either flank. He's a welcome addition to the squad."