Raytraced handles transparency a bit different (for i.e. glass and transparent plastic materials).
The transparency color should be regarded as a filter, where white lets all light through. This means that black lets no light through, no matter how high you set the transparency.
Since the Rhino color shows RGB values in the range of 0-255 per channel we’ll use that for the rest too - internally colors are mapped to 0.0-1.0 where Raytraced actually can handle values outside the range (for high dynamic range)
Now, if you want a black glass with 50% transparency you will need a value in the middle between black and white, that is RGB (128,128,128). That as your transparency color with transparency set to 100% gives you the black at 50% transparency.
If you want black 25% you set transparency to 100% for a color value of (64,64,64).
For a dark grey at 25% the maths will be similar, but you have some play room by setting transparency to something lower than 100%.
This all is a result of more physically correct rendering, and it will be the direction materials are going to for the future, at least for Raytraced, and when eventually the underlying engine replaces to trusty old Rhino Render.