Watching at your aberrant Natrix natrix, I think I've seen a "probably" similar blackish specimen on the island of Cres this year. I have pictures, but they are taken from great distance with a small compact camera, and you cannot see the details of the snake: however, my impression was that of a black Natrix natrix with somewhat strange over the body: "whitish" spots which at first I interpreted as "dirty spots" (algae or other things), but which have could well been white/hell spots just like those of your specimen. If you are interested, I'll post the pictures on my 3D about Cres (hoping the system will not crash as usual).
Other observations.
No sand boas?
Strange thing that, on some greek islands, the sand boas are very common, and that on Peloponnese you have not found any. I was in Peloponnese too (many years ago), and I've travelled in Greece, but I've never seen sand boas at all!
A question about the situla (yes: I'm a leopardsnake fan
): in which time of the day and on what occasion did you find the snake? Have you in situ pictures?