Offended? Well I doubt anyone would be offended if they have seen Morris Dancers (Btritish tradition) that routinely wear blackface as a disguise. Or even Africans that also wear whiteface as a disguise. The only people shocked are those that have a cultural reference that makes this shocking.  

 I also wonder about why the black servant  http://www.educationscotland.gov.uk/abolition/servantsinscotland/index.asp 
 suggests very strongly that this is the reason the character is black faced. And FInland has a similar aspect with Musta Pekka - a game.