Are there any hair dressers here who can help me?

I'm wanting to dye my hair to eventually a pastel blue, so I need to get my hair back to blonde.

I am a natural platinum blonde, and my hair is thin and fine.

The problem is, my hair is currently deep black.

Obviously I will be going to my own hairdresser to get this done, but I just want to know what the stages are, and if going from black to blonde is possible in one stage, or if it will take several weeks too do.

Thanks in advance!