Not to make the whole lot blonde, I mean, just the grey bits, which are about 5% at the moment. I wouldn't bleach first or anything. Has anyone tried this? I henna at the moment but a bit fed up with it. Had a couple of itchy/stingy scalp experiences in the salon so not wild about going back to dark synthetic dye. Am I right in thinking blonde dye has fewer problematic ingredients?
Do you mean permanent blonde dye? If it includes bleach (peroxide) you don't want it anywhere near henna as at best the results are unpredictable colour wise, and if you have used henna that contains metallic salts you could sizzle your hair off! ALL hair dye can be problematic with henna. I have been sucessful with darker semi permanents over reddish lush henna but it's not always even coverage. I'm not sure how you'd cover just the grey ?
If you went to a salon and got some woven highlights you could avoid the risk of stinging solution on teh scalp but they still might not be keen to colour over henna.
I recently went from henna back to dye (I was using henna for the same reason as you) and my hairdresser used an oil based permanent colour.. no stinging and decent coverage. It covered my grey but it was a slightly darker brown than I had originally wanted to even out the henna.