Your teacher was right to tell you not to use the phrase 'mixed blood'. Sometimes, an exact translation may be acceptable in one culture but unacceptable in others. This is not an issue of evidence - it is a question of appropriacy. In English-speaking contexts, it is appropriate to say 'biracial' , 'multiracial' or 'mixed race'.

Yes, 'mixed heritage,' is a very good suggestion.

Hi - here in the UK we can say that somebody is 'of mixed parentage.' Most commonly we would name the nationalities involved - for example 'He is of mixed Dutch and Chinese parentage.'

Hi 文静, That's a good question because it touches on culture not just language. In the UK we use the term "mixed race" for any person (child/adult) of a dual or multi-racial background, which is widely seen as a 'politically correct' label - this means that people feel it is a socially acceptable description. We also use "mixed nationality" if race is irrelevant (e.g. English/Swedish), or might state their heritage e.g. "she's half British, a quarter Chinese and a quarter French".

Another phrase is "of mixed heritage". It avoids words such as "race" or "blood", which can be culturally sensitive words. It really is a question of appropriate phrasing. Please, do not ever use "half-caste" - it's a derogatory word and implies that the person is racially inferior.