Black line last to turn a different colour (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

B (black) ROW (line) N (last letter in turn). As Bert solved 3d, Joyce solved 12ac – they clashed. Bert was correct and I had to accept that B (black) L (line) END (last) giving BLEND – to turn a different colour, didn’t quite work.