I've had a Powell/Whitefriars pat no 1258/8757 in black many years ago (which I wish I still owned - I had it for less than a day!!), and a posy bowl since which I think you'll find in Graham Hudson's photos on W/f's dot com. Further, if you look in The Country Seat's catalogue 'Glass Act II' you'll find a drinking glass with a black stem (cat ref E5). Oh, and Patrick's got some later post war, heavy, clear cased, pieces in black (published). Rare, but not unlikely......
Your keeness is to be commended
, but posy bowls were made by many companies, both here and abroad and are all based on variations of a wide rim with a bulbous base. The rim can curve, be straight or be anywhere inbetween. The bowl can range from cylindrical through to all but spherical. The shapes, and their combinations can
help identify which manufacturer made it. Hence my previous comments.
I can't see the 'ribbing' on the black posy bowl properly, but it looks more like rippling in the glass rather than purposeful ribbing made with the use of a mould when blowing (unlike the Webb version). Any comment, or a better photo, glassfreak? ....BTB, do you have a name??