Mono red goblins was working well, but adding black let me add Spike Jester (3/1 haste) in the two-drop slot. It also gices access to Fatal Push, to deal with Death's Shadow, Tarmagoyf, and other threats. Hazoret at the top of the curve gives me a late game threat in matchups where my opponent has been removing my goblins.  If I get a second Cavern of Souls out, I can name "god" as the creature type to prevent counterspells on Hazoret.

Shared animosity has been a nice finisher in multiple matches. My opponents are expecting another Goblin Chieftain (or Goblin King).  When swinging in with six goblins, each getting an additional +5/0, especially in Legion Loyalist's batallion ability is at play (all of my attacking creatures get first strike, *trample*, and can't be blocked by creature tokens).

Sideboarding in Blood Moon still works in most cases, since Spike Jester is typically on the field first. The additional Goblin King comes in in place of a Goblin Chieftain.

Shattering spree against Affinity and Swords.
Relic of Progenitus against Dredge, Flashback, Delve, etc.
Ratchet Bomb against Tokens, Affinity, Seismic Swan, and Death's Shadow. Against Death's Shadow, it can also hose me if I don't have my two-drop and higher goblins or goblin tokens on the field, but that's often still better than seeing multiple Death's Shadow swinging in.
Grafdigger's Cage against Flashback, Delve, and other graveyard shennanigans.

I had two Kologhan's Command in the sideboard slot, but found that the cases where I'd want to bring those in were often the same circumstances where I was bringing in Blood Moon. I want to find another red card to fill that slot--possible Dragon Fodder/Krenko's Command.