well that's her problem, you don't need to excuse it or lie.
You liked and still like legos, so your parents bought you some, so what?
Why should it worry you or make you think you "outed" yourself is what I don't get you can proudly be a woman who plays lego, just like many other women and your parents did the best for you buying things you like and not just stereotypical things.
When I was really little, I can remember myself playing with cows, with legos, with soldiers, with dolls and even a bit with cars!
I also have memories of boy friends and girl friends playing all those mixed things with me, I also have memories of people being like "that's for girls!".. I actually remember less "that's for boys!" selective memory or just more acceptance of variety on girls side? can't know I guess.
I won't be afraid to say I did karate when I might even like to go to martial arts again, actually, I found quite a bit of girls liking the idea but some just didn't find themselves participating in it.
Don't be so afraid.. You haven't done this to be the reality of the streotypical girl, you have done this to become the whole you.
Some gender boundaries are just imaginary while others are more real, you'll feel much better if you don't stress yourself over it.
P.S. I have a friend who plays with legos, does martial arts and plays computer games all day, she's not embarrassed of it and doesn't think it makes her any less of a girl.