Yea that's how it works, eliminate all the potential black jurors and you can get your client off.
Why?
Because black jurors cannot be impartial? Yeah. That's pretty ******* clear. But, you don't even know what the **** happened.
Take 48 potential juror.
They all randomly get a number from 1-48. They are seated in numerical order. Let's say that the 12 black jurors were seated at 3, 10, 12, 21, 22, 28, 31, 35, 38, 41, 44, 47. Both sides are going to strike the lowest numbered jurors because the first 12 (or alternate 13) remaining after all strikes are made get seated.
Prosecution strikes jurors 1, 7, 9, 10, 14, 18
Defense strikes 3, 6, 10, 12, 15, 21
They both struck juror 10. (happens way more than you might think)
That leaves 2, 4, 5, 8, 11, 13, 16, 17, 19, 22, 20, 23, 24
There's your jury. The defense struck 4 blacks, leaving 8, but the seating works out to exclude all black jurors but 1.
It's really not that hard to see it happening.