Because if oda chooses to go this route(bb with no haki), then this is his chance to further prove that with stronger haki you can conquor some of the strongest df abilities further backing kaidos words.
My personal belief is oda will go with all three(haki,yami,gura) As luffy toughest fight he will surpass all three.Sure his df ability will shine oda will never miss his joyboy oppurtunities but his haki will shine the brightest in this fight imo.
Personally i say no shanks haki would not grow as i am a firm believer that with black blade you need to make them yourself to get the full benefit. If you had said shanks turns his own blade black then yea sure that is him getting stronger so his haki would grow stronger as a result.