IN the first Traditional Bowyers Bible there is a section on glues where they talk about adding tannin to help hide glue resist moisture.  
Couple years ago I filled a 5 gallon kettle with shredded oak bark, topped her up with water and proceeded to simmer that for over 24 hours.  Eventually I boiled it down to a gallon of water, added a bunch of scrap sinew and rawhide and made a nice batch of glue.  Black as sin, thicker than snot.  It's been sitting in the freezer ever since.  One of these days I need to parcel it out in smaller batches and send it out to people that are interested in doing some glue strength testing.  If it works, cool.  If not, I have a big batch of unsweetened oak flavored Knox Blox.