HBO's filming in the summer, and the Bloodmoon working title was first revealed at the start of production. However, creating a logo for a working title is unusual, which suggests it could, in fact, be the Game of Thrones' prequel's actual title.
The Game of Thrones prequel series is set thousands of years before the setting of the hit HBO series, so the two shows won't exactly be directly linked. Still, the prehistoric setting is a period in Westeros called the Age of Heroes and it is when many of the Great Houses of Westeros (Stark, Baratheon, prequel series as The Long Night. Even A Song of Ice and Fire author George R.R. Martin uses that title, though he did clarify that The Long Night isn't the official title of the show.
And to be fair, Bloodmoon may still not be the Game of Thrones' prequel series' official title either, but the reveal of the Bloodmoon logo (which can be seen at Watchers on the Wall) makes an argument for this being the official title. The photo is a close-up shot of a production sheet, and what is shows is the word "Blood" above the word, "Moon" where the second "O" in Blood is reflecting to become the first "O" in Moon. Separating the two words is a spear, a common weapon in Westeros and one that dates back centuries given its relatively simple construction. It should be noted, however, that this is merely a photo of the image printed on a piece of paper in black ink, not a full color presentation or piece of key art for the series. But if legitimate, this would be our first look at the logo for the Game of Thrones' prequel series and a confirmation of its title being Bloodmoon (or maybe Blood Moon? Who's to say?).
Assuming Blood Moon is the official title of the Game of Thrones prequel series (possibly even in some horrendous branding, like Game of Thrones: Blood Moon, but hopefully not), then it's easy to see how this may still tie in with The Long Night. Originally, before it was a single evening depicted in Game of Thrones season 8 episode 3, "The Long Night", it was a period in Westeros history ruled by the White Walkers where, as Old Nan puts it, "the snows fall a hundred feet deep" and "little children are born and live and die all in darkness." The Long Night is basically a very long and terrible winter during which the White Walkers roamed freely, killing both humans and the Children of the Forest alike. It's possible, then, that an event like a Blood Moon signaled the start of this Long Night, not so unlike how the red comet in Game of Thrones signaled the rebirth of dragons - or the return of Azor Ahai, depending on who you ask.
It is from The Long Night that the fabled hero, Azor Ahai is said to have saved the world, so perhaps the Blood Moon ties in with that particular myth. It's already been speculated this Game of Thrones prequel series would dive into the origin of famous myths like of that of production has begun, it's only a matter of time before we learn more.
Source: Watchers on the Wall