Can a man say a brachah with just boxer shorts on, or does he need to be fully dressed, for example with a "double covering" like pants, and/or wearing a shirt, when I wake up and want to wash netilat yadayim? (Assume he is wearing a Kippah.)
The Shulchan Aruch rules (OC 74:6):
היתה טליתו חגורה על מתניו לכסותו ממתניו ולמטה, אע"פ שממתניו ולמעלה הוא ערום, מותר לקרות ק"ש
If his clothing was tied around his waist to cover from his waist downward, even if above there he is naked, he may read the Shema.
The operating principle here is twofold:
- One's erva must be covered.
- One's heart must not be able to see one's erva (were it to have eyes).
Thus, one could not wear a t-shirt and put a bag on his head, as this would violate the first principle. Additionally, one could not wear just a toga as this would violate the second principle.
Assuming then that your boxers are snug with your waist (usually due to elastic) such that your heart cannot see downwards, then you may recite the Shema and all other blessings (with the exception of Shmoneh Esrei for which you must look like you are approaching a king, ie with clothing on.)