My Cavendish has a rich flavor, and all the nicotine that it started out with. In steaming the leaf, it must be kept above the liquid in the steaming container. The residual water in the bottom of my pressure cooker, after many hours of cooking, is transparent and colorless.
As to using Cavendish for making a blend milder, it all depends on the variety of tobacco used for making Cavendish. If you use a mild, bland tobacco, such as some of the Maryland varieties, then yes, it will make a blend milder. If you make burley Cavendish, it will usually make a blend stronger.
My last batch of Cavendish, made from Bolivia Criollo Black, is full-flavored, medium nicotine, and smokes beautifully in a pipe as a straight, unblended tobacco.
So, select your variety for the nature of the Cavendish you wish to produce. I don't smoke cigarettes, but I would imagine that Cavendish from a flue-cure variety would be a likely candidate.
Bob