Q. If my minhag is not to say Baruch Hashem l’olam in Maariv (due to concerns of hefsek between geula and tefilla), should I also avoid saying Amein to this bracha?

A. Mishna Berura (66: 27) and Biur Halacha (ibid.) maintain that after reciting “Shomer Amo Yisrael Load,” after the brochos of shema in maariv, one is regarded as being bein haprokim. Therefore, Horav Shlomo Miler’s Shlit’a opinion is that one can and should answer amen on any brocho he hears.
The fact that one, following his minhag, does not recite that brocho, does not make that an inherently incorrect blessing, since the minhag of others is to recite.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a