This yellow legged thing on black and blue cochins is driving me nuts, sorry, hope this is not off topic. The question you asked was not completely answered and some stuff here is ambiguous.
According to the 2010 American Standard of Perfection, black cochin disqualifications of leg colour are (pertaining to legs and feet):
Black cochin
Disqualifications:
Shanks, other than yellow or black gradually shading into yellow
Bottoms of feet showing complete absence of yellow
Proper colour of legs and feet:
Shanks and toes:
Male -- yellow; bottoms of feet, yellow
Female -- yellow or dusky yellow, yellow preferred, bottoms of feet, yellow
Blue cochin
Disqualifications:
Nothing listed to do with leg and foot colour
Proper colour of legs and feet
Shanks and toes
Male -- yellow, bottoms of feet yellow
Female -- yellow to dusky yellow, yellow preferred, bottoms of feet yellow
There is nothing that I can find in the ASOP that talks about a leg colour disqualification in blue cochins. Yes, it talks about what the colour SHOULD be, but wondering why anything other than the yellow for males or yellow or dusky yellow for females is not listed under the disqualification.....this is not making sense to me at all. I am confused and I do not like to feel this way. It would be very nice if this could be cleared up, I am sure I am not the only one wondering about blue cochin leg colour, smiling. Have a wonderful day, CynthiaM.