hello m - my sympathies also in your current circumstances - I hope proceeding all go smoothly, and hope you are feeling a little more on top of things soon.

Whilst I wouldn't normally disagree with Lustrousstone

- as this lady knows far more about glass than me - I would consider on this occasion that Christine's rationale is wrong, when explaining that by definition malachite glass has to be green. If you read Cottle and Murray, they both discuss the various colours that were available in Malachite wares i.e. green, blue and brown principally - the use of the word malachite being simply to describe the marbling effect produced in the finished product (copying the natural substance) - not using the word green from the mineral.
For collectors the most common colour being the purplish malachite (blackberries and cream), and one of the least common was light brown, and both books shown named examples of the most common colours, in the illustrations. Other colours being black, grey and red.
Although most collectors often describe these variegated marbled coloured pieces as slag glass - I suspect most are probably simply opaque Vitro-Porcelain. Real slag glass was a product which I believe was produced in the earlier days of manufacture, and the floor sweepings, and dross from the coal or iron works was abandoned for a more controlled use of quality materials.
Your goblet is probably C20, and possibly from the States - Sheilagh Murray does show a figurine which she describes as "Marble Slag Glass" that has a two tone colour of white and purplish, and which she considers to be American, 'probably'.
From memory, I don't recall seeing anything in Slack or Lattimore in this type of two tone separated colourway - only the usual variegated/marbled looks.

The description of your piece might be......blue and white Vitro-Porcelain glass, American possibly

P.S. I've looked through Belknap's 'Milk Glass', but don't see any reference to Goats