Michigan State University Extension
Preserving Food Safely - 01600640
Whether or not herbs need to be blanched before freezing is a matter of some debate. The procedure for blanching is simple and fast. It involves gripping several stalks of the herb with tongs and quickly swishing them in a skillet of boiling water. The herbs are then spread on a towel to air cool. When cool and dry, herbs can be frozen.
Basil is decidedly superior when blanched. Chervil is not substantially improved by blanching, nor is coriander, because both freeze very well. Lovage freezes especially well without blanching. Blanched chives are sweeter, but unblanched chives are more oniony and have a better texture. Since chives are supposed to be oniony, it's better not to blanch them. Blanched thyme has a better color and a truer aroma. If you freeze thyme, freeze whole sprigs, rather than just leaves, to make it easier to handle. Dillweed is no better blanched than unblanched, and it freezes well.
While the color of herbs always improves with blanching, that is not reason enough to blanch herbs, since the color is lost in cooking. If you intend to hold them in the freezer for longer than six months, there is a distinct advantage to blanching herbs. In that case, blanching will make a decisive difference in their quality.