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As a local, perhaps I am bias but my understanding is that: Newcastle has always been portrayed as a stereotypical, white, working class city. Said generalisation is not exactly untrue.
It is quite unlike London, Bradford and Birmingham etc. in that there has been little immigration. I don't think that is particularly based on intolerance but simply down to a lack of opportunities, be it economically or otherwise, drawing in new populations. The majority of Scotland is pretty similar in terms of racial diversity. I am not sure that it detracts from the city it just gets annoying if you are looking for specialist goods.
However, there has been a notably change in the past 10 years and certain areas are becoming gradually more populated by Bangladeshi and African Immigrants. I would suggest checking out hairdressers in Benton and Fenham as they are most likely to cater to non-white residents. I have an afro and do find it difficult to get the necessary hair products, I tend to rely on the internet. All this aside though, Newcastle is, for the most part, a really friendly city and I hope you have a great time up here =)