He is somehow related to the person.

Maybe the speaker is pertaining to his ancestors.

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There maybe several interpretations to the line “I have never known you but you blood flows in my viens” on how he has never known the other person depending on the context of the poem, but it is of certainty that the relationship of the speaker with the person he is indicating is that of a blood relative. However, with that line alone, it is difficult to determine how close of a relative he/she is to the speaker – it could be a parent, or an uncle or a great grandfather.