Bluing is like soldering and using adhesives. CLEANLINESS IS ESSENTIAL. Use some solvent that leaves no residue, maybe acetone or lacquer thinner, to clean your Opinel blade before applying cold bluing. Hot water and dish detergent helps a lot, too. Also, wash you hands before and after bluing anything. You don't want even a fingerprint on what you plan to blue.
Something else that helps is to apply the bluing solution using a very small pad of 0000 (four ought) steel wool to work it into the surface you want to blue. Replace your pad of steel wool often and use only a small piece each time.
You might want to blue one side, get a fresh pad of steel wool, blue the other side, rinse it off under hot running water, very LIGHTLY go over it with a fresh pad of steel wool, and then repeat the process a time or two. Dry it and oil it when you are through.
Opinels are neat knives! You won't find very many folding knives with a handle that comfortable, either. And speaking of comfortable, be sure to check out what seems to be a new model, the Garden Knife, which is only six or seven bucks from eknifeworks.com (SMKW). You can't blue it because it seems to only come in stainless, but it has a spear point blade and a "slimmed down" handle. It REALLY feels good in your hand!