Although she does not say it in so many words, I think that the answer to this question is that Abigail blames the urge to dance on Tituba. What Abigail says is that Tituba "always sings her Barbados songs." When Tituba does that, Abigail says, the girls dance.
So why would Abigail blame Tituba? To me, it is because Tituba would make a great scapegoat. She is a person without any power in the community (as a black slave). She is already somewhat suspect because of the fact that is from Barbados -- people already think she does black magic. This makes her an ideal person on whom to pin the blame. People are likely to believe Abigail and no one is all that likely to defend Tituba.
Abigail, in a characteristic attempt to shift blame from herself and onto someone else, targets Tituba. This is a safe move, because Tituba is a black slave from the Barbados, who is known to have brought spells, chantings, and other assorted questionable practices with her. And, earlier in the act, Mrs. Putnam herself admitted to enlisting Tituba's help to use supernatural means to discover what evil spirit killed all of her babies. So, Tituba has already be implicated in evil-doing; why not add tempting Abby to dance and commit sin to the list?
When Hale turns the heat on Abigail and demands to know if she has sold herself to the devil, Abby panics. She knows that being a witch is a crime punishable by death, and she does not want that at all. So, she reaches for the most logical scapegoat, Tituba. She accuses, "She made me do it!" and then goes on to blame Tituba not only for the dancing, but for making her "laugh at prayer," drink chicken's blood, dream corruptions, and walk around the house at night naked. Since Abby probably has laughed at prayer and done those other things, and people have noticed, she is able to confess all of her sins and blame Tituba for every single one, in one single stroke. Quite malicious and ingenius, in an evil sort of way, that is.
I hope that those thoughts helped; good luck!