A lot of my favorite brand-name blends refer to themselves as "Mixtures", and a lot of famous shop "blends" are just mixtures of existing tobaccos or "blending tobaccos" from other manufacturers, so I think the distinction is just semantics.
That said, my favorite home blend is 1/4th Firedance Flake with 3/4ths Balkan Supreme. Very similar to Pembroke but more berry. I've had success keeping it "in house" by using G&H Balkan Mixture instead of Supreme. I imagine Squadron Leader would keep it even more SG but I can't bring myself to adulterate Squaddy.
I've also approximated Pembroke by mixing Cognac with BSOSM Match. I have also experimented with pressing, stoving and toasting.
There are levels to how "from scratch" a recipe is, but master blenders like Greg Pease use C&D blending components and processes instead of stripping the raw leaf with his bare hands. Same with Russ O and Sutliff, Hans Wiederman (HU) and K&K, Peter Heinrich and MacBaren, Mike Butera and Germain's, etc.. Same story with KBV, Watch City, etc..
Some "Master Blenders" are out there hand stripping and pressing raw leaf and others are at a desk mixing existing components, processes, and sauces to make the blends we love.