The key to using Black Orcs effectively is to restrain yourself from using them like their background info would suggest. This essentially means that, although the Black Orcs of lore will look for the baddest enemy unit out there to pick a fight with, the sheer fact that Greenskin elites are a lot worse than the elites of many other races means that in general you should not be fighting them with the Black Orcs. To avoid disappointment, your Black Orcs should stay away from units such as Swordmasters, Black Guards, Executioners, Chosen (even Chaos Warriors will prove troublesome) Hammerers and Temple Guard, since they will beat you down on a consistent basis.
That is not to say that Black Orcs make for a poor unit choice, not at all. In an army of unreliable and single-minded units, the Black Orcs provide reliability and flexibility, with several weapon choices, no animosity and excellent leadership. Instead of going up against the nastiest infantry unit the enemy can muster, you should make better use of their strong combat abilities, which are either extremely hard hitting S7 attacks, or plenty of S5 attacks with decent WS. If swinging their great weapons, they will make monsters of all kind nervous, even wounding Stegadons on 3+ in the first round of combat, and could even challenge the new Tomb King Sphinxes. The Black Orcs' immunity to psychology mean they won't miss a terror test either. Those same great weapons will also nullify the danger of Knights, who should go down easily against the Black Orcs.
However, not all armies will bring monsters or knights, but they will quite often bring units that are worse than Black Orcs in a fight, cheaper and most likely ranked up to be steadfast for some time. Units like that are usually not meant to win combats on their own, and fights between similarly poor units like Night Goblins versus Clanrats will most of the times result in an eternal standoff, with neither side winning by enough to make their enemy break from combat and flee. This is where bringing in the Black Orcs with two handweapons can be a good idea. Although they will tire and slow down after the first round, they should still continue to pump out plenty of casualties while their T4 and heavy armor should prevent a high amount of casualties from their own ranks. In a matter of rounds, that big steadfast unit will be steadfast no longer.
Since Black Orcs are fierce fighters and a reliable unit, they are one of the units least in need of a fighting character to lead them. That said, animosity is less of a problem overall, meaning you won't always need your Black Orc characters to keep order in the ranks. That can sometimes allow you to add your fighting characters to the Black Orc unit for an even 'arder unit. A good choice would be the Black Orc Big Boss with the Battle Standard, who will add an additional two attacks to your front rank with higher Weapon Skill in addition to his other benefits.
Unit size: 20-24 make for ideal sizes of Black Orcs - enough to survive and defeat a monster, or survive long enough to break cheap, steadfast units, while not big enough to become unreasonably expensive. As Black Orcs rely on their attacks, rather than their ranks, to win combats they should almost always deployed 6 or 7 wide, except if fighting monster to their front where you'd want to make your ranks last long enough (since you can't fit in more than 4 bases against a 50mm monster anyway).
Equipment: Shields are quite cheap and with so many S5 template war machines around, those shields can make a difference, as it makes it possible for the Black Orcs to survive their direct hits a bit better. If joined by a character, be sure he's bringing the Iron Curse Icon to increase the likelyhood of you surviving the artillery fire aimed at you.
Command Options: Musicians are extremely useful and a bit of must have for most combat units. It's the same with the Standard Bearer, who will add to your vitality score for a quite realiable unit, and increases your combat resolution. A Boss will cost you even more than an additional Black Orc, but does add an additional S7 attacks early on. Still, it's a bit on the expensive side and so only worth it if you're bringing a character in the unit.
Magic Banners: Although they removed all of the extremely useful magical banners of the old army book in the new incarnation, bringing a magic banner for your Black Orcs is still a bit of no-brainer. The Banner of Eternal Flame is the obvious choice, a clear-cut favorite since their flaming attacks can either strike last at S7 or at the initiative of 2, meaning no matter if it's a Hydra or a unit of Trolls, the Black Orcs will take them down. Other solid options include the Banner of Swiftness to make the Black Orcs march across the field quicker than ever, and the Standard of Discipline makes for solid break tests even if away from the general, or creates a LD: 10 bubble if you plan on bringing a Warboss General on foot with the Black Orcs, while pretending that his name is Grimgor.