Word Distinction: Mixing Things

Mix is the most general word to use about susbstances:
to mix butter, eggs and flour
Blend is often used about the action of mixing in careful proportions to produce a particular taste, smell or other good result:
to blend spices / blended whisky
Mingle is usually intransitive and is used
a) of people:
mingled with the crowd
b) of flowing liquids of different origin, colour, temperature, etc.
The fresh water of the Amazon mingles with the salt water of the South Atlantic.
Merge is intranstitive and is used when one thing becomes lost in another, or two things become one:
an insect that merges with its surroundings
the place where two roads merge
When two or more things combine, they join or stick to each other, but keep their own identities, and may be separated again under suitable conditions:
Hydrogen combines with water to form oxygen.