This rather minimally named effect is the frei0r.B effect from Richard Spindler.
This is down as "Extract Blue from Image". But to me it removes all colour and I get a grey image.
This effect appeared in Kdenlive ver 0.9.3 in the Color group. There are also frei0r.G and frei0r.R filters that extract Green and Red - but they are not in Kdenlive - dunno why.
Removed from Kdenlive ~ ver 17.04