When did BlackStar and Tsubaki swap?
I doubt there will be evidence to back this up, however when Black Star says "Tsubaki.. Ninja-to mode!" and a burst of smoke appears (around 16:27), is practically the only chance they could switch - and it makes perfect sense.

Within this puff of smoke, Black Star makes his 'escape' to a better position, and instead of Tsubaki changing into Ninja-to form, she becomes the dummy.

I doubt that Mifune saying that his eye colors changed was quite true, or at least can't be verified by the viewers, as Black Star doesn't have colored eyes - just typical black dots. Tsubaki, on the other hand, has the 'other' style of anime-eyes, (which are a shade of blue). 

The important part is, however, that when Tsubaki is in Dummy-form, her eyes were the same as Black Star's, as proven when the dummy 'floats away' (it is very clear to see the eyes changing from Blackstar's to Tsubaki's).