There are many "Black" marbles, most are Deep blue, brown or green, some may be purple.
Yours is a deep blue imho.
Do not get me wrong here this is just a very nice corkscrew. ( very nice)
Ron Shepard had evidence of a black marble glass recipe--I have seen no evidence as of yet, but if he saw it--I believe it.
Black glass in any form--not just marbles is very rare.
Cheese is right--this is a close as it gets for me too.
Nice Marble--they do get blacker--
Backlight them all and be rewarded.
Marble--On!!