I’ve had both the 6x and 8x versions of the Kowa YF. While a great little bin for the money especially I’d never compare them to my Kahles Helia 8x42’s (SLC’s) or my Leica HD+’s. Still an excellent recommendation for kids. Never directly compared them to the Blue Sky II’s..
I currently have them all- 6x30 and 8x30 YF, Bue Sky II 8x32, SLC 8x42, and UVHD+ 7x42. I've compared them all side by side, and even did some again today.
To my eyes both YFs have better sharpness and contrast in the CENTER of the view, compared to the Blue Sky. Almost as good as the SLC and UVHD+. I know that sounds crazy that a $200 bin can have CENTER image sharpness and contrast close to an SLC and UVHD+, but it's just simply true, maybe I have cherry samples. It is not as good, but it is surprisingly close. The Blue Sky has a larger sweet spot and edges are better than the YFs, but center of view sharpness and contrast is not as good.
The focuser on the Blue Sky is way better than the YFs imo. It is smooth and the right amount of tension for my liking. The eyecups are better on the YFs imo.
For me eye placement is easier on the Blue Sky than the YFs.
Overall obvioulsy the SLC and UVHD+ blow away the others. They are both arguably some the 2 best bins ever made.
IMO the Blue Sky is OVERALL a better bin than the YFs, but again the CENTER of image in the YFs is special, especially for that price range. Also, the more 3D like view the porro prism YFs produce is a bonus.
I enjoy the IMAGE through the YFs more than the Blue Sky, but I enjoy using the Blue Sky more than the YFs... if that makes since.