Are you using pure body art quality henna? If SO, you should be able to chemically color it with no problem. I would take a picture of your natural har color to a GOOD colorist and tell her you want to go back to that.

If you are trying to do it naturally, I have no advice. Sorry :( Red is a hard pigment to get out, if you manage to fade it, its still going to just be a lighter red and you will most likely have roots.

Are you *sure* you don't want a pretty rainbow on your head? (I'm cultivating mine! *s*)

Though I haven't and don't want to fully grow out my henna, my experience of letting it grow out over five or six months is that:
a) it doesn't fade significantly if you don't recolour - it might drop back to a less vibrant red, but it'll stay red (this is better, imo, than chemical colours that fade to disconcerting shades of pastel pink or salmon or something, and it's why I went back to henna after trying the supermarket stuff for a while)
b) the regrowth line is not much of an issue for me over my dark hair, but it's startling on my grey. I suppose dark blonde will be a lot worse than the one; and not quite so bad as the other. Still not something you'd like to live with until it grows right out, and I'm guessing you don't want to cut it short either.
c) I'm using henna and cassia now - I want to keep the red tips, so I've gone back to using henna at the ends, but with cassia at the roots it's certainly possible to achieve a well-blended colour gradation. It's a rainbow instead of a horizon, but it's still a rainbow...

So, if you totally want to avoid the dark tips; light roots effect I suspect you'll have to chemically lighten the ends. And of *that* I have no experience to offer, sorry.