How do I play TBC Blackrock?

I just saw a post saying that "I've just discovered that the Blackrock TBC realm is out". How do I play it?

i was logge in yesterday, but when i try logging today. the buttong was gone.

how i got in there the first time was go to blackrock realm for wotlk, then in bottom right corner was a button for swapping to tbc, i pressed that and boom there was blackrock tbc i logged setup char. but when i wanted to log back in today, the option is gone rip, not sure if its just for me