Certain cultivars are darker than others, but there are many KBG/TTTF mixes sold commercially, and many cultivars are recommended by the seed producers as mixing well with TTTF. Stay away from the darkest elites like Midnight, Solar Eclipse, Blueberry, etc. You can find which ones are darkest via the last NTEP trials for KBG (www.ntep.org). Another thing to keep in mind is that KBG needs lots of iron to really be as dark as it can get, so if your soil doesn't have high irons levels the KBG won't be as dark as possible. I am planning on dormant seeding my TTTF with KBG this winter as well, but I am picking the most shade tolerant KBG I can get.