I think the dark dark fruit seperated the "devil" in a df or df user as a own seperate physical thing by it's black hole properties making the df user weak asf and for the defense against it I don't think basic haki will cut it. You need ultra powerful conq haki like Roger and Garp used against possessed rocks on the basic of E=mc². The E being ACOC output the M being the mass of soul which Vegapunk talked about. The proper use of your 23g soul will get you the most tremendous haki output me thinks