The reason some do not saying amen to the chazzan's bracha just before Shema is because it is a hefsek between the brachah and Shema. Based on the Gra and the Mishna Berurah, it is not clear if it is a hefsek birchas hamitzvah or birchas hanehenin. If, however, we labor under the assumption that the brachah is a birchas hamitzvah on the Shema itself, then it seems there should be no issue of hefsek when davening outside the zman since the mitzva of the Shema itself isn't even being fulfilled.
I received a psak to not say amen to the chazzan's bracha just before Shema even outside the zman for Shema; apparently not everyone agrees with the logic above. I didn't get a reason so I decided to toss the "why" over to this community:
Why not say Amen after birchas ahavah when davening outside the zman for Shema?
The mitzvah is not in effect at that time; and, compounding the problem, when discussing the Shema of Shacharis, the mitzvah of Shema has already been fulfilled (assuming that the davener said Shema during the zman).