Long story short, I could not find a navy blazer :P So I ended up wearing a navy button down top with 3/4 length sleeves, grey pants, black shoes.

The interviews went great!

Excellent!

I got an offer and am in the process of negotiating, we'll see how it goes

if you're not comfortable in a dress shirt, i suggest dress pants, a tank and a cardigan. if you feel that looks unpolished, i suggest a belt to nip you at the waist, and black shoes/ballet flats.

i work in a business casual environment and i rotate between dresspants/cardigans or dark denim/boots/blazers. i also meet with clients.

I think the choice you made was excellent. Grays and Blacks you can't go wrong as it gives a nice conservative professional look. I have sat on many interviews and "casual" doesn't mean quite as casual as some folks take it as. I still prefer an attractive business look until such time you are actually hired as part of the team and can dress as they do.