If your hair is dry and damaged do not bleach it. Use the colour b4 and shampoo your hair a few times then apply a semi perninant ash toned dark blonde/light brown as colour b4 will usually leave you with orange/ginger hair. Then use a reconstructor (either aphogee, joico kpak, dumb blonde) to help repair the damage followed by a deep condtioning treatment.
If you are wanting to go blonde, wait a month and if your hair is feeling better you can dye it blonde or bleach bath it then tone. Make sure to soak your hair in coconut oil for about an hour before bleaching or applying a permanant dye as it will help prevent damage.