Baking Soda. Place your blade in a pan w/ just enough water to cover it. Add two table spoons of baking soda. Stir it around enough to dissolve the baking soda and spread it evenly in the water. Let sit overnight.
Next day, use a soft bristle brass brush, or an old tooth brush to gently scrub the pitch/tar away. Rinse the blade, dry it, and you're ready to go. If I'm not going to use the blade right away, I usually apply a light coat of WD40 and store it.
I've been using this method for years and now buy 5 lb. bags of baking soda at Sam's Club.
Bill