Another one who's heard good things about the colour b4. Now, take the following advice with a pinch of salt, as I'm not overly fussy about hair colour being totally natural looking (I've been all the colours of the rainbow in the past) but when I went looking for colour b4, I couldn't find it, so had to improvise.
Like you, I had been dying my hair a vibrant red for a good while (I'm naturally a warm chestnut brown). I then dyed it a very dark brown, almost black. Then got bored and decided I wanted to go much lighter.
I couldn't get a hold of the colour b4, so got a pre-lightening kit (loreal and garnier nutrisse do them) and used that on my almost black dyed hair. It went through a few different colours as the dye did it's work (you're basically bleaching the colour out), and after about 45-50 mins I got it to a very light yellow/orange colour.
I then put a dark blonde colour over it after a day or two (a garnier or loreal one) and hey presto - hair looked perfect! A slight auburn tinge where the worst of the dark dye had been in my hair, but as it worked out it looked like a professional highlight job, as I had light blonde, dark blonde, and some auburn tones running through my hair.
Another caveat - my hair is currently short enough, above my jawline (I also have a tendency to go for drastic changes in cuts
) so if things went haywire on me, I wasn't too fussed about destroying a full head of hair that was hanging halfway down my back. If I had still had that same length hair I had over a year previous, I'd have been a bit more hesitant TBH.
As it was, if I'd made a haims of my hair, I would have just cut it off even shorter, and left it get back to a healthy state and grow it out a bit again. As it turned out, my hair is in great condition, and the colour looks pretty darn natural and professional.
Sooooo - if you're feeling a bit daring, you could try the pre-lightening kits to lift out the worst of the dark dye, and given that it's a brown colour you have in at the moment, your hair will lighten up in to auburn colour, rather than a baby-blonde cololur, so could be very close to your normal colouring. If you do decide to try this, just make sure to lash conditioner in for a while after you do it to make sure your hair doesn't go all dry (some people's hair has a tendency to go a bit strawlike with bleach, mine seems to be pretty resilient).
Once again - the caveat is that this worked great for me, but only you know how much of a battering your hair can take, or how much you're willing to try
edited to add: two different hairdressers I know were both pretty impressed at the results, and were a little surprised I managed to lighten it so dramatically myself, and had said they generally would not attempt to do something like that on a client in case it went wrong and they'd get into all kinds of hassle over it, so that might be why hairdressers won't do anything for you.
But both said that my hair was in great condition and recommended if I wanted to lighten it even further at some stage, to use a home-highlighting kit to put in some more blonde, so obviously they don't think home kits are the work of the devil if used properly