some of the hirsustissimum / esquirolei are quite dark, but the darkest by far are the vinicolor forms of callosum, a.k.a viniferum. Note, there are 2 types of vinicolor callosum, now so interbred in the hybrids and with each other that these days it would be hard to get a 'pure' seedling of either form. There is the 'JAC' line of vinicolor callosums, this is the form Ramon imaged above, later described as the species viniferum. It is different looking than callosum. The second strain is the 'Sparkling Burgundy' line of callosums, used by Rex van Delden when he was employed at Stewart's Orchids in California. The 'Sparkling Burgundy' line looks identical in shape and form to a typical Thai origin callosum, including the typical relatively few on the petal margins warts and no spots or warts in the central portions of the petals. I have a 'Sparkling Burgundy' type from an F2 from seed, but I damn near killed it 6 years ago and it hasn't recovered yet. It is still alive, and now within a year or two of blooming, but it is not a very vigorous plant.
So these are the two callosum strains to look for if you want the darkest of the purple vinicolor species.
Paphanatics is the first place I would go if I were to try and find these 2.