What is the best nourishing oil / cream for damaged blonde hair?

You're not going to like my answer, but the fix for badly damaged hair isn't something you put on it. It's scissors.

That said, if you can't or won't cut the damage off, I've been more than satisfied with L'Oreal's El Vive Total Repair 5, which comes in a jar. You apply it to clean wet hair and leave it in for five minutes or so (longer doesn't help), which is a fine time to shave your legs, pumice your feet, whatever. Rinse thoroughly.

It seems to help minimize further damage and to hide existing damage more than hair products that cost four or five times as much.

No product fixes damage, though, no matter what they say. A split end stays split.

I like pantines minute miracle deep conditioner, you can leave it in for as long as you want to then rinse it out.  I also like proclaim deep conditioner and Aphogee protein treatment for damaged hair. I used to use VO5 hot oils for damaged hair, but they didn't do much for me. 

The best thing for damaged hair is to get it trimmed off. Then seal it with a leave in conditioner the next time you wash your hair. 

John Frieda Sheer Blonde Flawless Recovery Deep Conditioner for Damaged Blonde Hair.

A good hair moisturizer conditioner and shampoo for starters. You could try getting the ones you could leave on for a couple hours then washing it out and see if that helps.