i boil: 50/50 red wine (cheap, just for acidity and color) and water or grape juice + lots of bags of hibiscus tea

and mix this into:
ca. 100g pure henna (imported body quality, or zubeda from local indian store, if spontanous)
+ 20-50g (depends on mood, i like those irregular black-ish streaks)
herbal henna "black" w/indigo (from health store brands, eg logona)
+ 10g ground cloves

i either let this sit for 1 hour to wait for dye release (strongly recommended with yemen henna   or put it directly on, to keep the warmth on my head - i didn't notice much difference in the final result. as i've got lots of gray hairs at the top, i always start there, using a hairdresser's color brush to make sure i cover every streak, and then, once the top is covered, drop the brush and use my hands to smack it all over the rest, starting with a ponytail, and ending in some sort of muddy turban. i cover it up with a plastic cap, then some foil to keep the heat, some old towel, and leave that on for 4-5 hours.

the result is a very nice dark red on my naturally dark blonde hair, even my grays are covered nicely. and i also love the effect of adding the indigo/herbal mix. it doesn't seem turn my hair browner or black, just darken the red and add some very dark streaks, in a rather punk-ish manner which i like a lot  

elektra