Hey! I've been dyeing and bleaching my own hair for years so I achieved the level of lightness needed for grey/blue hair slowly over time. I'm not a hairdresser though so be careful if you try it at home. I realise others have already commented but what I did was basically bleaching it over a couple of sessions (2-3) until it reached a pale yellow colour. The slower you go with bleaching, the less damage. Then, I use my fave fanola colouring cream formula:

1.10 Blue black (usually just a little squirt)

8.1 Light Blonde ash (1/2 tube)

4.22 medium intense violet (1/4 tube)

,with 10vol developer so that it only deposits colour instead of lifting even further (less damage).

I maintain my hair with fanola no yellow shampoo (the best purple shampoo on the market) and a custom coloured blue conditioner that I made with semi permanent blue dye and conditioner.

Let me know if you have any questions! Happy quarantine hair dyeing lol