Ok well with most people talking about how this benefits a lot of attacking moves, I felt like this was time for me to take a unique point of view on this and discuss how this helps utility based moves. Most utility based moves aren't going to provide coverage in the normal sense of the word, but they can make for some strange sets and provide walls with ways around opposing walls / offensive mons.
Mew @ Leftovers
Ability: Synchronize
EVs: 252 HP / 224 SpD / 32 Spe
Careful Nature
- Will-O-Wisp
- Soft-Boiled
- Taunt
- Knock Off
While Will-O-Wisp still can't burn Fire-types, making it a Dark-type move means Mew no longer gives free Flash Fire boosts to Heatran. While this isn't a big change, Mew still appreciates no longer having to take Flash Fire-boosted Fire Blasts or Lava Plumes from Heatran.
Klefki @ Leftovers
Ability: Prankster
EVs: 252 HP / 4 Def / 252 SpD
Careful Nature
- Thunder Wave
- Spikes
- Magnet Rise
- Play Rough
Making Thunder Wave a Dark type move means Ground types can no longer switch in freely and "tank" a Thunder Wave as it is now a Dark-type move. With Ground-types no longer being able to switch in, Klefki now can be an even bigger nuisance than in standard play with only Electric-types being able to switch in (since they are naturally immune to paralysis). This change means old checks such as Excadrill and Garchomp can no longer get around Klefki as Excadrill loses its Speed in Sand and Garchomp now becomes slower than Klefki, allowing it to use Magnet Rise shenanigans.
Excadrill @ Leftovers
Ability: Mold Breaker
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Rapid Spin
- Iron Head
- Earthquake
- Toxic / Stealth Rock
Excadrill now not only has an unblockable Rapid Spin, but it can hit the Ghost-types that normally spinblock super effectively! But seriously, giving Excadrill Dark Rapid Spin means it can now get rid of entry hazards more easily. It also has a nice Toxic Spikes immunity, thanks to its Steel typing, making it probably the most reliable spinner in Custom Power.
Cobalion @ Leftovers
Ability: Justified
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Volt Switch
- Stealth Rock
- Close Combat
- Iron Head
While not OU, Cobalion is one of the few viable Pokemon that has access to Volt Switch yet isn't an Electric-type. Giving Cobalion a Dark-type Volt Switch means it can now pivot out of Ground-types that may tend to wall it. Dark-type Volt Switch means that Cobalion no longer fears a Ground-type blocking its pivoting or being able to fire off a free STAB move as it forces Cobalion into a different check.
Honorable mentions:
Breloom @ Life Orb
Ability: Technician
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Spore
- Bullet Seed
- Mach Punch
- Swords Dance / Rock Tomb
This allows Breloom to get around Grass immunities, such as Sap Sipper, and not powder immunities, which are not very prevalent in OU, but it is still a neat idea.
Ferrothorn @ Leftovers
Ability: Iron Barbs
EVs: 252 HP / 88 Def / 168 SpD
Relaxed Nature
- Leech Seed
- Stealth Rock / Spikes
- Gyro Ball / Power Whip
- Power Whip / Protect
Same vein as Breloom as this change allows Ferrothorn to get around Grass immunities, but they aren't too prevalent in OU.
While most utility moves don't benefit from a type change, these select few can give Pokemon new niches, further their own utility, or avoid benefiting the opponent's Pokemon. This metagame definitely benefits offensive moves, but don't count out utility moves! n_n