From Robertson, Studies in the Text of the New Testament.

We are confronted with an early controversy over the meaning of John 1:13. Some of the early writers accused the Gnostics and Ebionites of tampering with the text and changing the reference from Christ to Christians. Today it is replied that it was the other way, that those who believed in the Virgin Birth of Jesus changed the plural to the singular in order to get a proof text. So there we are. The Greek manuscripts give only the plural, and it will take a good deal of clear proof to get around that fact. Hort argues, however, that there are some 65 cases in the New Testament where all the Greek manuscripts are wrong and where we have a primitive error. So we are confronted with the possibility of that being the case in John 1:13.

Once the singular appears in John 1:13 one must admit that it suits the context admirably and leads right up to the statement of the Incarnation in verse 14. There is something, then, to be said for the use of the plural (bloods) instead of the singular, for according to the Virgin Birth idea Jesus had no human father and there was no union of male and female blood. There is no denying the pertinency of this reading, once it is admitted.