Blend is stronger. "Mingle" is often used to describe social mixing, such as at a party. You could use "commingle" to describe the same, but where two different groups are involved. For example, there might be a group of tennis players and a group of swimmers at the party and you could say that the groups "commingled". I don't recall seeing the word used in the way you described, e.g. mixing substances.

Commingle is a very rarely used word. If in a technical context I would say 'mix together' or 'blend.' Although if the ingredients are being dissolved into solution, it would be more precise to use the term 'dissolve.' How I would express it if it involves dissolving: The ingredients (or components) must be dissolved in a liquid solvent, such as water, to facilitate reaction. If this is about cooking and/or not involving dissolution: The ingredients must be mixed with a liquid solvent, such as water, to facilitate the mixing and reaction process. 'Facilitate' can be replaced with 'to allow for.' Could also say 'with a suitable solvent, such as water.'

Blend is suitable in this instance.

I agree with what Matt T says about 'commingle' being rarely used. 'Blend' is a very common, everyday word that more or less everyone in every English speaking country understands and uses. So I don't agree with your suggestion that they are interchangeable. 'Commingle' might be more common in certain parts of the English speaking world, or in certain contexts. It would require an in-depth study of the corpus to give a definite answer, but it seems to be used in some slightly more technical contexts or where a slightly more formal lexicon is used (such as literary criticism). Therefore, yes, it's a synonym, but really I would advise you to use 'blend' unless you are certain that you are communicating with people who have a high level of English (I imagine that quite a few native speakers won't understand what it means).