So Mono-Black Devotion is far more resilient than Mono-Blue Devotion, but there are still numerous ways to beat it (again, I piloted a version at Gameday).
First off, very aggressive decks are actually quite well suited against it. Many of its avenues of play require the payment of some amount of life (think Thoughtseize, Underworld Connections, Erebos, God of the Dead) making the job easier for a deck that is fast enough to punish them for reducing their life total. To this end, red deck wins, Selesnya/Boros/mono-white aggro are all actually not half bad. Sure, they have spot removal, but no board wipes, so their ability to deal with 3-4 creatures is considerably reduced.
Second, the deck can beat control, but it has some trouble. Specifically, if you can manage to get Elspeth, Sun's Champion into play and keep her there, she will simply beat the deck. With no way to deal with repeat instances of the tokens, it has a hard time. But more generally, it's a pretty predictable deck, so it should be relatively easy to pick out what to counter/remove and what not to. Mind you, the threats are pretty diverse, so even an unattended Pack Rat can take over the game.
Third, remember that black has ZERO enchantment or artifact hate, so to that end, Assemble the Legion or even Detention Sphere will really screw them up.
Fourth, you REALLY need to disrupt their card advantage. This means dealing with Underworld Connections and to a lesser degree, Erebos. If they cannot draw additional cards, well, odds are you can beat them when they run out of removal or otherwise stop drawing gas.
The one thing I can say is that if you plan to just join them, I would suggest adding some added flavor to the build and diverging a bit to catch them off guard. For instance, I personally enjoyed running a list with 4x Pack Rat, 3x Nykthos, Shrine to Nyx, 3x Whip of Erebos, and 2x Abhorrent Overlord - the last one of which could easily win mirror games for you.
Anywhom, it's a good deck, easily more flexible and capable than Mono-Blue Devotion, but by no means unbeatable.