With lots of time you can theoretically achieve a deep indigo blue using woad. It contains indican, the same blue dye as found in indigo.

The problem with woad is that there is very little indican in it, and to get it out you need to ferment the leaves. Usually woad will give you a pretty but pale blue (like faded denim), or bluish-grey.

Most web photos of woad dye results have been colour enhanced. Just saying.

This second image is more true-to-life. Image source: Woad Dyed anche un sacco di grandi informazioni su questo sito.

Il guado veniva coltivato ampiamente in Inghilterra e Francia prima che l'indaco entrasse improvvisamente in scena dall'India.

Storia del guado in Gran Bretagna di Teresinha Roberts