Let me start with a basic definition, courtesy of dictionary.com (see, I cite my sources like a good girl)
blended family :
a family composed of a couple and their children from previous marriages.
Notice a key term to the definition, "their". Blended families are not defined by drawing lines in the sand, stating "his" and "hers". Rather, its an all encompassing term, specifying a new family formed by including both parties AND THEIR children.
Granted, things are complicated. More often than not, both biological parents are still in the picture to some extent, and often, one parent may object to another party stepping in to fill this role. Of course, for the case of this blurb, I'm not talking about replacement parents. More specifically, I'm talking about the social dynamics of a family structure who live within the same dwelling, where one of those parties is not a biological parent.
Regardless of your position on the subject, certain facts remain true. Whether intentional or not, the outside party will have an effect on your child(ren). Anyone with regular contact to your kids, whether teachers, friends, babysitters, extended family, etc. have substantial influence on your child(ren). I could cite sources here, but in the interest of keeping this reasonable in length, I'll let you do your own research.
When we tell a partner that we do not want that person taking a parental role in our children's lives, we are clearly defining boundaries as to how far we are willing to accept him/her into our own life. Consequently, we put that person in a very difficult position. By living in the same household and engaging in family activities, that person already has a great deal of influence over our children. However, what we are restricting is their level of authority, not their level of influence.
Instead, a proactive approach would be to discuss at length your personal view on parenting styles. Protecting your children isn't about shouldering all responsibility and authority. Rather, coming together with a person who shares your personal values and is willing to present a united front will provide the best outcome and the most stable home life.