It is an amazing performance. People around Blaine can't believe their eyes as he spits out one frog after another.
Watch the trick here:
We see no other possible explanation: before the filming began, Blaine had to swallow those frogs. It’s not clear where exactly the frogs were or how he got them back up. He probably used some sort of bag containing the frogs which he could slide deep into his throat. The retrieval mechanism for the frogs remains a mistery. He probably had a high-tech solution to deal with that part.
There is no way he could use sleight of hand for this. We studied the video in detail and the frogs clearly come out of his mouth.