So I've never played around with black infect (just never got around to it, to be honest), so take all this with that in mind.
1) I would probably drop blue or make it more pronounced. You aren't really running enough of it to justify a second color.
Blighted Agent is nice, but you aren't even running 4 of them to help pull one. Looking at what you've built, you might be better with mono-black; you could remove some spells and add more blue, but then you'd need to fix your mana again. Mono-black, especially for a cheaper CMC infect deck, could probably safely run 20 lands, giving you four more spells to play with.
2) Most infect decks I've seen try to get something on the board and make it big, fast. This usually involves green, but even if you don't want to run green, you could add
mutagenic growth for a quick buff to help put you over the edge. Costs two life, but if it wins the game, two life doesn't mean anything.
3) If you want to control the board,
doom blade is a cheap removal (with a small drawback).
Murderous Cut is another solid addition, especially if you know you'll have cards in your graveyard to
exile to make it cheaper.
Victim of Night is another good mono-black removal (that has a drawback.)
4)
Lashwrithe, especially if you go mono-black, is a pretty good addition, as it can buff your creatures pretty quickly and can be re-equipped for life rather than mana you might need for something else.
5) This is an obvious one, and I assume you aren't interested in dropping ~$180 for four of them, but
inkmoth nexus is pretty much a staple in infect decks.
Again, I haven't tried black infect (I have a budget blue/green one that's pretty run-of-the-mill, without the
inkmoth nexus, those things are pricey.) but that's probably where I'd go with it.