I did some quick checking:
Not as much known about bite blocker, which is my usual objection to this sort of thing, why on earth people think that "natural" or "herbal" means non-toxic is a complete mystery to me - and every other toxicologist in the world. This is actually a fairly complete summary document on what little is known. The NOEL is a little higher than DEET, but it hasn't been studied as much, it is also a NOEL (No observed (adverse) effect level) only for what has been decided to be the active ingredient. (Higher is better).
http://pmep.cce.cornell.edu/profiles...k_mcl_0810.pdf
After this brief assessment, I wouldn’t worry about bite blocker, either. I personally prefer DEET because it’s the Devil that is known.
Ticks are huge here, I sit in the driveway and check my dogs over before we go in the house, then myself. I found 22 on the belly of my doxy-x once!