Blood and Wine, the second and final Witcher 3: Wild Hunt DLC, will be out on “07 czerwca 2016,” if the Polish gaming site GRYOnline is to be believed. And that, according to Google translate and WCCFTech, which captured a screen of the listing before it was deleted, means June 7.
That's far from official, of course, but it does fall within the “first half of 2016” window that CD Projekt has been bandying about since late last year. It's also very near the end of that period, and since the studio cited that target as recently as a couple of days ago on Twitter, it's not unreasonable to expect that this leaked date is probably at least close to the mark.
Blood and Wine is set in the previously unseen realm of Touissant, a nation within Nilfgaard that harbors a dark and bloody secret. CD Projekt says it will add another 20 hours of play to The Witcher 3: Wild Hunt, but aside from that, and a pair of screens it released in December, it's remained entirely mum about what's coming.
I've emailed the studio to ask if this date is accurate. I don't expect anyone to confirm it one way or the other but on the off-chance it happens, I'll let you know.