When I was blonde (about 7 years ago) I did attempt to my own hair a few times. The results were me dying my hair an interesting shade of orange and having to get the hairdresser to sort it out and on another occasion after I'd been doing it myself for a while, I was told my hair was developing a 'greenish tinge' when I went to get it cut and agian, had to get the hair dresser to get it sorted. My sister has slightly darker hair than me and she dyes hers blonde all over and again, the times she's done it herself it's just not looked anywhere near as good as when she's been to the hairdresser and had it done properly.
Personally, I'd always go to my hairdresser.