I'm still waiting for those non-racists reasons for blacking-up.

I can well see how blacking up could be used to highlight or enhance the offensiveness of some act of racial hatred or discrimination, but to the best of my admittedly limited knowledge, morris dancing has never had that aim. I'm assuming Motley do primarily it because they want to. That is a non-racist reason I can certainly live with. I expect if they start getting significant negative feedback from black people at gigs, they'll think again, but if their 'gimmick' is not motivated by racial hatred, discrimination, mockery etc., then I don't blame them for their stance in this sorry incident.