When Trudy scolds Dorothy, asking her what kind of man would jump into bed with her dearest friend's husband, Blanche defends "not necessarily a bad person. . ." However, in Season One's "That Was No Lady" Blanche professes she has never been with a married man because he always gives the expensive gifts to the wife.
To add: Agreed! Also, Blanche has said something to the effect of, "Why do men lie about being married?Don'tthey know we'll sleep with them anyway?". This alsoimpliesthat Blanche has no qualms about sleeping with married men.