Is there any problem with reciting a b'racha (or other devarim shebikedusha) underneath a second-floor bathroom?
There are two issues with bathrooms learned from parashas ki seitzei (Machlokes Rashba and Rosh). The first is v'haya machanecha kadosh- which disallows you saying Hashem's name when you are in the same room or within 4 amos of tzoah or its smell. The second is v'lo yerae b'cha ervas davar- which disallows you from saying Hashem's name when your eyes are in viewing range of tzoah. We are machmir for both shittos. (succinctly stated by the hakdama to the mishna brura 79)
So if you are 4 amos away from the tzoah/smell+outside the bathroom and looking in the opposite direction, you can make a bracha. Certainly downstairs from a bathroom. (I recall a shittah that considers the toilet being halachically equivalent to tzoah, but can't remember where that is or if it applies to our toilets.)