It absolutely will help. It doesn't need to be anything fancy. Any blanket that would keep you warm in bed will work, even better than blackout curtains since a blanket will have a good R-value.
Blackout curtains like others have recommended are not great insulators. They simply have an opaque, rubbery, white backside to reflect and block sunlight. The will block currents of warm air from moving between the rooms, but a blanket does that, too, and provides a higher R-value.
I live in an RV van and had blackout curtains between the front and back of the van for years before I decided to also add a quilt, and it made a huge difference in keeping the back warm in winter, and keeping the sun-caused heat out of the back in summer.
When I lived in an apartment I used to have a blanket between the kitchen and living room. The living room had an AC unit, and the kitchen had.... an oven. The difference the blanket made was astronomical!