I'll try to help out if I can.
My theory on the Opal/Blanc de Lait, white/opalescent debate.
If you look at the 1878 pattern book:
http://www.victorianpressedglass.com/pdf/sowerby/sowerby_1878_colour.pdfThe first 3 or 4 pages show patterns in Turquoise, Jet and Opal. These were the first colours made in opaque Vitro Porcelain which was patented in 1876.
Opal must be opaque white because Blanc de Lait (opalescent) was not patented until 1880.
Slack agrees with this but Cottle does not. I think Slack is correct.
The pieces on the websites referred to by Flying Free are, as Mike said, Opal with enamel decoration. I think this is cold painted as it can be rubbed off.
So for me these websites have incorrect descriptions. Depends which book you are reading.
The Turquoise frosted piece is in the 1882 catalogue and is acid etched. Sowerby used both acid etching (possibly for smaller items) and sandblasting, if you look at the Sowerby Colours page:
http://www.victorianpressedglass.com/sowerby_coloured_glass.htm towards the bottom is an ice bucket which has been sand blasted.
In the pattern books Sowerby sometimes refers to this as 'obscured'.
The biscuit jar in the V&A is 'stippled', very fine dots in the mould. Sowerby used this decoration on a number of pieces.
Also, towards the bottom of the Coloured Glass page I try to explain the words 'Stained Blanc de Lait' on the front of the 1882 catalogue.
Stained Blank de Lait possibly refers to green tinted BdL and possibly the pink tinted BdL Gladstone Bags. Cottle uses the term 'Rose Opalescent' but I'm not sure where that came from.
Mike has a very nice pair of green tinted BdL swans on another thread.
I don't think the jug or Venetian piece on the NZ Glass Museum website are anything to do with Sowerby. The only prunts (raspberry) I have seen are on Sowerby Venetian glass.
Sowerby Venetian is interesting, I have a number of pieces and I am trying to put together a web page about this at the moment.
Mike