Board Thread:Speculation, Theories, and Rumors./@comment-11144873-20140108072813/@comment-4220590-20140121194538

Word of God says Blaze is from the Sol Dimension. So the most plausible explanation in my opinion is that Blaze made an unexplained visit to the future of Sonic's world where she then teamed up with Silver. Then, when the timeline was rewritten, Blaze returned to her homeworld. The rest is history.