Black's discard in blue/red:
- Blue tends more towards eating up your opponent's useful draws by manipulating their library than straight up discarding.
- I could certainly see a blue card with something along the lines of, "Enchant player. Enchanted player's maximum hand size is reduced by 5." That would be a sort of indirect discard. At the same time, I could also see that in red, oddly enough, although red's version would be more like, "Each player's maximum hand size is 2."
- Red's take on offensive discard is more like card replacement. They don't really reduce the resources your opponents have available, but they do ensure that said resources aren't safe just because they're in hand. Wheel of Fortune and Wheel of Fate have everyone discard their hands then draw 7 cards, for example. Barbed Shocker and Collective Dissonance make an opponent discard their hand and draw that many cards. Burning Inquiry makes all players draw 3 cards, then discard 3 at random. Lots of things like that.
Black's creature kill in blue/red:
- As Sinfire Titan pointed out, blue's got a bit of a polymorphing theme going on.
- Blue also gets some creature kill by freezing and shattering things via
Frozen Solid,
Ice Cage, and the like.
- Red kills creatures, as it always has, with damage. That's mostly with direct burn spells, but also some creature fighting (either literally like Magus of the Arena or just via flavor with Fling).
- Red also has some straight-up creature kill, mostly in older sets, but it tends to be expensive and have secondary effects.
- Red's got total board wipes of various sorts.
- Red really doesn't like blockers. I can see something like, "Destroy target blocking creature."
- Red could also synergize with its damage. "Destroy target creature that was dealt damage this turn." is a purely black mechanic already, though, so that might be color bleed. Maybe just dealing a large amount of damage instead of flat destruction (ex: "THIS deals 10 damage to target creature that was dealt damage this turn.").
Black's life steal in blue/red:
- Not many ideas for this. The life loss side is easy, red's got lots of direct damage to face. It's the life gain side, which is otherwise only really in green and white.
- Blue has a little bit of pseudo-life gain through illusion/delusion with Delusions of Grandeur and Delusions of Mediocrity. I'm not counting some of the really old, obscure, and color-hating cards like Lifetap and Merchant Ship.
- Red is really the opposite of gaining life. Like, completely and totally, 100% against life gain.
- It might be cute to have a horridly convoluted life steal in blue. Make something like Delusions of Grandeur (toned down to something like +/- 3 life), with an ability built into it that gives it to another player. If it has cumulative upkeep and the ability that passes ownership can be used by anybody, it's more of a hot potato sort of deal, which fits into the red side of things, too.