A blessing that targets all friendly units inside 12"
And that took bugging multiple Eldar for the wording as it's not in the main Codex, and apparently paying for another full Codex price just to get Iyanden isn't that popular around these parts. As it thus is
targeting instead of 'saying otherwise', the Psyker runs into the Declare Target rules on Page 67 and can only target himself, his vehicle, or another unit in the same transport.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.