There are a couple of "Magic" jars.  This one is the more common but in this color it is very rare and highly desirable.  The color is often referred to as black amber but mine is as dark as many other jars which are referred to only as "black glass".  There are other jars which come in what is described as "black amber" but for some reason, the black amber Magics seem darker than any of the others and this is the darkest black amber Magic I've seen so I don't think it would be at all out of line to describe this as a black glass Magic.

This seems to be a distinctive color rather than some sort of "mistake".  If it were a mistake, I'd expect to see similar mistakes in all shades between the usual light amber through to this almost opaque amber but as far as I know, there are light amber jars and almost black amber jars with nothing much in between which suggests to me that this was an intentional coloration.  I have no idea why this would have been done, but that's wnat the evidence seems to suggest.