If you have been colouring your hair dark purple for a while then the hair maybe stained quite deeply. Staining happens when dark or intense colours are applied over and over until the cuticle becomes very heavily saturated with artificial dye. Bleach should remove it but I think you will get a very warm strawberry pinkish type blonde the first time you attempt to lighten it.
If the hair is going to be dry and damaged do you really think it is worth it? I would ease off colouring it for a while and let the colour fade and let some of your natural colour come through. Have some decent hair cuts to get rid of the existing colour. This should make it easier to go blonde in the future.