The NWT I use sys "[] Brackets enclose words inserted to complete the sense in the English text; [[ ]] suggest interpolations in the original text."
Now I'll quote from the Zondervan NIV Study Bible. The note on that part of the verse says: his own blood . Lit. "The blood of his own one," a term of endearment (such as his own dear one)," referring to his own Son).
Hmmm, just read 29 and 30. Interesting stuff. A warning about elders/overseers/pastors who wil arise and distort the truth in order to draw away disciples after them. But I'm sure that could never hapen here...