Yes - IF any of your customers/clients did anything illegal. Proceeds of crime, KYC etc. I mean, think about it - what search terms would you optimize on? "Best weed in the market?" "Buy your murders here?" ... Good luck trying to explain away that you had zero knowledge of working with 'shady' customers. IANAL so go ahead if it seems to anyone to be a good idea.

Or perhaps you meant operating as a sort of entirely Dark-Web Only SEO operation? If so, they already exist.