some people find kissing in public offensive, some people find breast-feeding in public offensive, some people find Big Issue sellers offensive, some people find street preechers of whatever religious affilation offensive, some people find homosexuality offensive, some people find cross-dressing offensive and so on infinitum

That's true, but it doesn't actually support your position (that what matters is whether blackface causes "significant offence").

Let's say that a neighbour with rather, um, old-fashioned views says that he's offended by my failure to discipline my wife, who persists in speaking when not spoken to. By your argument that's only one person, so it doesn't matter.

Let's say that 1,000 different reactionary people tell me that they're offended by my failure to discipline my wife, etc. A thousand people is a lot of people - by your argument that's "significant offence", and I should listen to what they're saying. Right?

To me, the number of people offended doesn't much matter. A million people can tell me they're offended by my fondness for beer, and I won't change my ways* - because partiality to beer isn't something that I think it's appropriate to be offended by. But racism is something that I think it's appropriate to be offended by, so if somebody accuses me of using racist imagery I will think long and hard about whether they might be right - even if (perhaps especially if) I think they're wrong.

*Actually quite temperate ways, let's not get this out of proportion. "Moderation is true temperance" (Wm. Yates, and he should know).