There's surprisingly little at the moody end of the spectrum.
Hilsea Lines as has been suggested, and you could try accessing the derelict Greyhound Stadium and landing craft nearby.
Some of the staircases and alleyways around the Civic Offices give off quite a wrong-part-of-town vibe but there's usually heavy police presence on account of all the drunk students, at least some of whom are known to take a **** in the very dark corners you'd want to be shooting.
Other than that, there's pockets of darkness around the city with no particular vibe, and short tunnels at varying distances off the island. Not posting the location here though - PM me.