Semi permanent 'very light brown' has left my salt and pepper hair with a 'blackish look' - help

I've been through this too. In my case, I used a very gentle product--one of the "won't burn your eyes, ammonia-and -everything-else-free product that just wasn't strong enough to get the dark hairs lightened and the grey hairs permeated. I had better luck with a stronger product. Semi-permanent dyes, in my opinion, are best for minor color changes. If you want something better and more consistent, see a pro. They really do know how to do the job better than the OTC products, believe me.