Always go with that card's oracle text, which is its official rules text. Burnt Offering has both a red and a black mana symbol in its oracle rules text, so it has red/black color identity, and so it can only be used in a commander deck, if the commander has both black and red oin its color identity.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)