I have made single varietal ciders. not my favorites. In my experience it is more reliable to get the apple juice mix tasting right before the fermentation, so you can adjust the percentage of different apples to try to get the best mix of flavor sugar and ph. before the ferment
OTOH I can see how the blending process could be interesting depending on your apple options. Most apples I can get are not good for single varietals. There are a few, Stayman, Pink Lady, maybe Albemarle Pippen. The rest need some sort of balancing from other apples. YMMV.