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As far as I'm aware there are no names available for colour varieties of S. selenitica.
In S. pyrastri, var. unicolor has been found to occur only in females without ovaries (publications by Van Doesburg 1964, 1970 and Knutson 1971). The cause of this is is unclear, but might have something to do with parasitism in the larval or pupal stage.
In certain genera, e.g. Platycheirus, Melanostoma, Pyrophaena, melanism seems to occur mainly in so-called 'intersexes': specimens with characters intermediate between male and female (eyes not touching but appoaching more closely than in females, postabdominal segments without male genitalia, but nevertheless 'thicker' and more asymmetric than in females).
In many other syrphids, 'melanistic'individuals are just the dark end of a whole range from very dark to very pale individuals. In many cases this is correlated with temperature during the pupal stage (publications by Dusek & Laska, Mart Ottenheim, Jonathan Heal). This is why specimens in early spring, cold summers, at higher latitudes or altitudes tend to be darker than specimens from warmer places.
I think the latter mechanism probably applies to your specimen of S. selenitica (from a high altitude...). It's a dark specimen, but the shape of the markings on the tergits is still visible, and the distribution of other dark and pale specimens is more or less as in normal specimens, albeit the dark parts are more extensive. This would also explain why the dark markings on the sternites are wider than you would expect. Similar kinds of variation in tergal and sternal markings can be noticed in e.g. Dasysyrphus and Eupeodes.