"Apparently the firm distinction of blue for boys, pink for girls is quite recent.."
-Yes...and horribly ubiquitous.
" What do you mean about pink having 'no transparency'?"
Nearly all colours can be made transparent (-think stained glass) but pink was not really createable without directly mixing with an opaque white agent.
If you see pink in a carpet for example, its modern and done with similar flourescent dye technology that gave us dayglo.