Hi - Welcome to the GMB

I'd suggest this mark is the result of the inclusion - as can be seen elsewhere in the glass - of a metallic foil of some description that was applied randomly whilst the glass was hot, and the appearance at the base just happens to be where some foil came to rest as the glass cooled, and can now be seen due to removal of the pontil scar.
Gold, silver and perhaps other metallic foils have been used as a means of decoration in and on glass for some decades - particularly by the glass houses at Murano. IMHO I doubt it's mica, a mineral, used extensively earlier in the C20 for radio valves and other thermal connections, and for which transparency was an important requirement - whereas foil, in particular gold and silver can be made ultra thin and flexible.
Sorry, no idea as to the origins of your vase, hopefully others here will be able to help - the neck appears v. narrow for a vase. There were some difficulties, apparently, in producing a real black glass, and most pieces that appear black are a very deep purple when held up to transmitted light - as you've discovered - though true black I think can now be made.