You'll need to do some custom validation, then, based on whether the field is blank and another field with some value that you can use to try to determine user intent (i.e. -- user clicked submit button when the field was blank) instead of using the built in "can't be blank" option on fields.
I'm going to have to say, though that while I understand where you are coming from, this isn't an approach I agree with. At one point, I also didn't like marking fields as invalid if the user hasn't had a chance to fill them out. In the end, though, I've decided that having a red asterisk display on the "can't be blank" fields is what people are used to -- lots of online forms are that way, and it is actually easier to fill them out, because I'm not left guessing what is required.