Tamblyn revealed the news in an interview during an appearance on VH1's "Big Morning Buzz Live".
"You're still very close with your co-stars and the three of you are godmothers to Blake Lively's baby - is that a fact?" television personality Nick Lachey asked the actress. "Yes, that is a fact," Tamblyn confirmed, sending the audience into applause.
Lively, 27, and actor husband Ryan Reynolds welcomed James in December 2014.