Nobody said just because it's a custom it should be immune from criticism. However, if we are going to criticise something in a different culture we should perhaps look at a bit of context first. For the "blacking up" example Wullie made, why should that be seen as racist in Spain based on something that was happening in a different country with a different language and culture years and years ago? For the Bilbao players referring to Inaki Williams as "darkie" why is that any different to nicknames based on something else in a person's physical appearance? It's only different because we have been taught that it's different, there is nothing inherently wrong with a nickname based on the colour of someone's skin, no moreso than a nickname based on somebody's height, etc.
I mean there definitely is a difference. No one was ever hung or persecuted based on their height. That comparison doesn’t work at all
To someone who is from an English speaking country it doesn't