Deskdiary - What you have here is the difficult situation where intention collides with perception.
You did not intend any racial slur with your costume, but that does not mean that none was received by people who saw you wearing it.
This gives rise to the difficult moral dilemma - are you entitled to wear what you want, given that your motives are not provocative, and by the same rule, are people entitled to take offence at what you wore, regardless of whether that was your intention or not.
Personally, my way round it is this - I would say that you did not intend to upset anyone, but you ran a knowable risk, and as such, you have to accept the consequences of your actions.
Is it wrong to assume that someone who blacks up is racist? Yes it is - but such is our society's perception of that action, that you knowingly run that risk, and lack of intent does not present an inviable defence.
I am sure that in future, you will consider carefully your costume choices based on this experience. Having a hero, and not intending offence does not mean that everyone else is going to understand and accept your appearance. You need either to be ready to accept that, or re-think your outfit for the evening.