Post from Narutoforums who basically write what I already did in spoiler thread:
"Judging from how some of the giant arms and legs started out looking normal:
Then gradually turning black as they are grabbing Maria:
Then becoming fully black as she is getting clutched:
"
Looking like haki for sure. She literally make the white hands black when she use her attack, if it was a usually black shady colouring Oda would not draw the hands with white colour and start to make them black in the moment when Robin use her attack.
In case everyone can see that Robin turn out the white hands and covered them with black hardening and then finish off BM.
Their would be no point for Oda to make white hands and making them black if Robin didn't use haki.
"Judging from how some of the giant arms and legs started out looking normal:
Then gradually turning black as they are grabbing Maria:
Then becoming fully black as she is getting clutched:
"
Looking like haki for sure. She literally make the white hands black when she use her attack, if it was a usually black shady colouring Oda would not draw the hands with white colour and start to make them black in the moment when Robin use her attack.
In case everyone can see that Robin turn out the white hands and covered them with black hardening and then finish off BM.
Their would be no point for Oda to make white hands and making them black if Robin didn't use haki.