Very dark but jazz suitable is a tricky ask. The darker a mpc is, the less flexible its tone and the less you can really push it volume-wise. Eg: Caravan, the ultimate dark classical equipment, gives basically one tone quality, and won't let you vary it. Rascher is a little more flexible that way, but still keeps the lid on.
I might recommend a Morgan C (for classical) model. They're dark (with slight edge), good volume, easy-speaking and less resistant.
Well, I'll take a shot at it. There's the Mouthpiece Cafe Expresso 7*, which is pretty "dark" and based on the Selmer soloist but it has decent projection. I tried it and liked it. Also Saxscape Extra dark but I'm not sure if they are available right now. Check his website. I'm sure there are others. Perhaps you might check out getting a stock HR Otto Link refaced to blow better.
Hard to suggest anything without knowing your experience level. What horn do you play? What mouthpieces have you tried? Everyone has a different opinion as to what is "darkest". What is dark for one may not be dark for you. So much depends on how dark you naturally tend to play.
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