I could briefly explain why Balkans have an overlap with Georgians and North Caucasians. I won't include the geographical and climatic similarities between Balkans and Caucasus. That's pretty obvious already if people take a look at the satellite map or check the weather lol.
Balkans have a lot of ANF. North Caucasians and Especially Georgians also have a lot of ANF. Which is only a piece of the puzzle about why these people have an overlap in terms of look.
Balkans have a lot of steppe. A sizeable chunk of steppe is CHG. North caucasians have both, a lot of steppe and a lot of CHG independently (at least going by their G25 CHG scores). Georgians, besides the ones from Mtiuleti, Khevsureti, Tusheti who have similar levels of steppe as North caucasians there are Eastern and Western Georgians who don't have a lot of steppe itself but they have a lot of CHG, especially the westerners. CHG in itself was probably hardly distinguishable from modern day Balkan people in terms of looks. Hence we see such an overlap between Balkans and Georgians & N. Caucasians.
Finally, I would like to touch on the Natufian admixture in both groups. Balkans on G25 rarely ever get any Natufian and Georgians only get a few percent of it. In many cases North Caucasians are completely lacking Natufan admixture. Natufian Hunter Gatherers were sand people from SW Asia with dark complexion and overall exotic features. The more Natufian admixture a group of people has, the darker they will look. That's the general rule. Considering the fact that people from both the Balkans and Georgia&N. Caucasus have very little to no Natufian admixture pulls them close together ln the looks spectrum.
Overall, this is how I could briefly explain the similarities between Balkans and Caucasians. I could write more but I don't have time for it.