It's a dot, with a footnote saying "Tarnost", anything beyond that is speculation. Tarnost could be the name of a natural landmark for all we know.No, it appears on one of Tolkien's maps (HOME).
Authors very frequently change their minds when writing, so i would say if Tolkien didnt include any background to "Tarnost" in any later works (which he didn't) then it wouldnt be any sort of significant city.