There are more foreigners in Japan than you might think. People there are used to the idea and not interested in blonde hair at all. If they do look at you it won't be because of your hair. If you are in an area like Harajuku or Shibuya you'll find that a lot of Japanese people there have blonde hair anyway. I have dark blonde hair and have lived in Tokyo for 12 years, and believe me, no-one cares about my hair. If I do get a second look it is usually because of my height (5'9", 175cm) as I'm taller than the average Japanese woman, and people often look to see if I'm wearing high heels :)

You'd probs get the same "dude needs a haircut" look that you'd get anywhere else.

I totally agree that you shouldn't get any problems :D

When I went to japan, my host family invited their piano teacher over to meet me and the first thing she did when she saw me was rub my hair and tell me I was blonde to test my japanese XD

But she was pretty quirky, so I don't think you have anything to worry about :)

If you're a gaijin(foreigner), your gonna get stared at anyway.
I'd wear that hat though.
You are going to be going to one the most homogeneous societies in the world, and the older generations consider young people who dye their hair blond and brown to be "delinquents" (although I understand that that those feelings are subsiding somewhat in the major cities.)
You don't have to scurry around hiding your hair, just know that you WILL get some attention no matter what.

If you ever see Fruits magazine there are quite a number of girls that have their hair dyed blonde.

Caucasian people are not as unusual in Japan as you may think. No-one is going to stare at you just because you have blonde hair, it is just not interesting enough to any Japanese person. If you have a blonde mohawk, then maybe.