I don't think there was one actually. Sony sold an RGB cable on the Japanese market for PS1, but I don't think there was ever an official PS2 RGB cable available here or anywhere else in the world. Maybe I'm mistaken...
Yes in a cardboard box, I have one. I'm not sure about it being sold in a plastic blister too though.
They didn't sell an official PS2 RGB lead because of the whole DVD playback bullshit where one of these leads gave you nothing but a green screen unless you used a workaround disc or similar.
Nowdays you are only going to find it second hand... In europe they release official leads for the PS1 and PS2 (but not the PS3 although the PS1/PS2 leads work fine). The part numbers for these were... Playstation 1, which was coloured Grey as the original PS1 - SCPH-1052 Playstation 2, which was black and quite a chunky lead compared to the PS1 one - SCPH-10142 In Japan the RGB 21 Pin lead was SCPH-1050, there is no PS2 RGB lead although they did give it a part number of SCPH-10120 (and not SCPH-10140 as you would have thought) as it was mentioned on various press releases regarding DVD playback on the PS2.
