1. Each time that a person hears a blessing, when he hears the person making the blessing pronouncing the Name of Hashem, he should say “Baruch hu uvaruch shemo”. The exception to this is a blessing which he is listening to in order to fulfil his own obligation (e.g. the blessing on the blowing of the shofar etc.) where according to the opinion of the majority of the halachic authorities he should not say “Baruch hu uvaruch shemo”. In the event that he did say, one can be lenient to assume that he has fulfilled his obligation. (Shulchan Aruch and Mishne Berurah 124)
2. However, when he hears a blessing at a time when it is forbidden for him to interrupt, (e.g. during the verses of praise, and obviously during the blessings of the Shema etc.) he should not say “Baruch hu uvaruch shemo). (Shulchan Aruch and Mishne Berurah 124)
