To blanch fresh cilantro/coriander, bring a large pot of water to a boil. Have a big bowl of ice water ready.
Once the water is boiling, dip your bunch of cilantro into it just until it wilts. This shouldn't take more than a few seconds. You don't want to cook out the flavor of the herb, just kill off those decomposing enzymes.
As soon as the cilantro has wilted, immediately transfer it to the ice water. This eliminates the residual heat that would otherwise continue to cook it.
Pat the blanched cilantro dry.
Strip the leaves off the stems and transfer to freezer bags and freeze.
Tip: spread the leaves thinly in the bags and store flat. This will enable you to break off just what you need when you want to use some - but not all - of a bag of frozen herbs.