I live in CA. We are covered under Blue cross HMO plan which does not cover fertility treatments. I would like to know which insurance covers it.

I do not know of any that will cover it. Normally fertility treatment is considered an "elective" treatment and not medically needed.

Also since you have already been (treated) for this, another insurance that even would maybe cover it, would now see it also as a pre-existing condition.

Thirteen states have a mandate to some extent for Infertility treatments. CA has a mandate to offer, the carrier has to tell the employer that infertility coverage is available, at additional cost.
Fertility LifeLines™ - State-Mandated Insurance List (http://www.fertilitylifelines.com/paying/insurance/statemandate.jsp)
Check here for more info.