Forget the Scotchguard!  Go buy yourself a gallon of Thompson's Water 
Seal; yes, the stuff for wood decks, etc.  Then, spread the blanket out 
on the floor, or a piece of plywood over 2 saw-horses (that works better 
if you aren't up to crawling around on the floor!).  Get a big paint 
brush, dump some of the Thompson's into a small container and paint 
away.  
This has worked on any blanket that is NOT a 100% nylon exterior.  The 
nylon won't absorb the Thompson's but any natural or part natural fabric 
will very well.  I've used this on New Zealand rugs that I washed in the 
washing machine and then dried in a dryer.  I just don't think that 
field washing gets them clean enough after a whole Winter!  So, I drag 
them to the laundromat in the Spring when the horses are through with 
them and wash them, dry them, then paint with 2 coats of Thompson's.  
Have also made some cheap canvas blankets and a couple of cloth ones, 
including a Baker blanket, waterproof the same way.  Always use 2 coats, 
let dry in between coats, and the blankets will last all season.
Have used this too on Gortex stuff that had lost its waterproof 
capability after one too many washings.
Hope this helps,
Lyn Kamer, C.E.S.M.T., and my Mustangs, the girls: PC and Star
The Healing Touch of New Jersey