Question: If you have a steak medium to rare, such that it's still pink in the middle, is it kosher? At what point is meat cooked *enough*?

Answer: 
I'm assuming you are talking about steak which has been processed under rabbinical supervision.

Presently, in the US, with the exception of liver, all kosher meat is salted to remove problematic blood by the certifying agency before it hits the stores and restaurants. So meat which is edible does not have to be cooked "enough" and rare - or even steak tartare (raw) - is fine.
Raw, unsalted meat is completely kosher. Just rinse and eat. Shulchan Aruch Yoreh Deah siman 67 siff 2.

EDIT. The Shach in siman 76 s.k. 2 also point out that unsalted roasted meat even if only rare is also permitted to eat.
The point of my original answer was to dispel any misconceptions about blood, but this edit will answer the assumed question more directly as pointed out in the comments. 

One more point of clarification as pertaining to cooked, not roasted meat, is after salting the red juice is not considered blood,but called mohel, as pointed out by Monica. This is the Ashkenazi law lichatchila. For Sfardim however, if possible they should be putting the raw salted and washed meat into boiling, not warm or cold, water in order to seal in the red juice which the Rambam did in fact consider blood, and the Shulchan Aruch says to take into account, when possible. See siman 69 siff 19. This is process is called chalita. 

I don't know any Sfardim who actually do this, whether at home or in restaurants (though some Teimanim do). So for all Acheinu bnei Hamizrach please comment or quote any modern day rulings, but this is what the Shulchan Aruch says and some people reading this question and these answers might find this interesting.