Well, I'd have to say early 1965 for a very weird reason. When I look at my mother's 21st pictures virtually every girl in the pictures is wearing a version of this dress. Apparently they were all raving about what they were going to wear, and when they all turned up they were all in the same style. Different dresses, but all black knee length dresses with an hourglass waistline and all with sheer sleeves. So there must have been some sort of a trend going on. I haven't seen much of it in the 60s mags though.
The nearest thing to it that I can find in there are some dresses in various issues of Queen Magazine from 1962 and 1963. The 1962 dresses are inclued in a feature on learning how to dance the Madison and are mainly Jaegar and Harrods, but they don't have the high neck or the sleeves. The 1963 pictures are all on debutantes, so they'll be all Harrods and Hardy Aimes sorts of things I guess. Still not the sheer sleeves though, just the dress shape. So I guess these pictures in the magazines must have been an inspiration or the later dress that you have found.
The only really gothic trend I can think of nearest that time was late 1970 - early1971 when all the dark black velvet came in, dark straight flat hair, blood-red lipstick and black eyeshadow, really decadent darkly coloured fabrics, thick black chokers at the neck, scarves worn at the waistline, all of that sort of thing. Ringo Starr's first wife latched onto that style the moment it came in and she didn't let go of it for years. She's my absoloute clothing idol, but I haven't her figure, hair type or skin colour so I can't carry it off. *sob!*
Anyway, this dress is deffinately too pretty for the gothic look that came in, but it could easily have been in the black-dress-fashion-parade at mum's party (LOL! Poor mum) and is very much in the shape worn in 1965/1966 films when they have a "party" scene.
The bodice reminds me of the dresses that were inspired by the "topless dress" scandal but I can't remember which magazines those dresses were in so I don't know where to start looking it up. :( But that was mid sixties too as it came along with Gernreich's 1964 "topless bathing suit".
I'd really say though that it's a mid sixties, upper middle class style for the 18-24 age bracket.
I think it's *divine*.