I know that the "blue" essential oils are supposed to be good for skin ailments, but I can only think of blue chamomile and blue gum eucalyptus.

Are there any other blues worth considering?

German (Blue) chamomile is best known for the high blue azulene oil. It is only produced when chamomile flowers are steamed for chamomile essential oil. The unsteamed flowers have no blue colour until the oils are steam extracted. This is why Blue chamomile is better as a topical skin treatment than the other chamomile types, (Roman, Moroccan) which excel in aromatherapy.
That said, there is some azulene produced by Roman chamomile, yarrow, blue tansy, valerian, blue gum eucalyptus and peppermint, but in lesser strengths.

Are any of the others any good, and do they have particular uses that the chamomile doesn't?

(Thank you for your patience with my 600 questions.) Where on earth did you learn all this?

Also, does gentian have azulene, or is that another compound?

That got me thinking, and it doesn't appear gentian blue is likely to have much azulene. (I know it was used medicinally, because in older pet care manuals, its use was recommended.)

I've ran across some interesting things:
*Elecampane root does contain azulene, and many more compounds that I ever knew of.
*There are plants I can't find the common names for - example: Libanotis intermedia.
*That the genus Achillea has essential oil uses in skin care.
*That coleus has medicinal uses. (C. Forskohlii, to be exact.)
*That artemisia essential oils have uses other than pest repellency - as in skin care. A. arborescens, to be exact, in particular a Pacific NW varietal. "A. arborescens oil has the highest amount of chamazulene of any essential oil known." http://atlanticinstitute.com/artemisia.pdf#search=%22es...taining%20azulene%22

Reading stuff like that makes me really realize I will probably be ignorant of most useful plant properties my entire life. How on earth does one keep up with all of this?

I started with wondering what else the plants in my garden could do, besides eating or looking at them. Starting from there, botanicals not growing in my area. The secrets they hold! When you read or hear that a certain plant has been in use by a culture for thousands of years, you know there has to be something to it. It is only in the past 100 that science has started to investigate and breakdown why these things work, so there's alot of information coming in all at once. I did not know about the artemsia, for instance, although I have applied poultices of 'Old Man' to boils and abcesses with success.

I had a conversation once with ogkitt about how does one choose which school of thought with herbals. Do you try to take in a bit of them all, Chinese, East Indian, African, European, South America, North America original cultures all employ health care systems based on native plants. They usually end up doing the same thing in the end, but which one does it best, fastest? Which one to choose?

I don't think, unless it was something you devoted most of your life, that a person could keep up with all the medicinal benefits, the Latin, the methodolgy and history of botanicals. Some people do get in on this stuff real early, and make a professional career out of it; I wish I was one, and I envy people like our young ChrisJ, with a full lifetime ahead, a innate curiosity, intelligence, to explore these things to his heart and mind's content. We can only encourage them. Who knows the vital secrets they may end up discovering? Chinese cucumber, for instance, is under study for cancer and HIV/AIDS treatments. Within 10 years, they may have a solution to stop the dying from these diseases. Who would have thought that a lowly vegetable could save millions of lives?
I guess it's like anything else these days, in the Information Age. You need to pull out what you need, and leave the rest alone until you need more. Having the internet is a blessing and a curse, for research and the bombardment it provides.