Henry Fonda was roped into this -- he had a higher box office rating than the perfect actor for it, Gilbert Roland. There aren't many roles both of these men could play, and this wasn't one of them.
The story concerns the Spanish Civil War.
The script was written by an avowed Communist, John Howard Lawson who wanted to "present the Communist position" in his scripts. He doesn't really get to do that in Blockade, since it's deliberately ambiguous as to the different factions, referred to as "they" and "us." The costuming also doesn't suggest anything as far as sides.
The story concerns a place called Castelmare, where Marco and Luis (Fonda and Leo Carrillo) help a Russian woman, Norma (Madeleine Carroll) who has had a car accident on the way to her father's. For Marco, it's love at first sight.
When war begins, Marco is the head of a group of peasant attempting to defend Castelmare. Meanwhile, Norma and her father are forced to spy for the other side. Marco winds up killing Norma's father.
Castelmare cannot get any supplies, and Norma is being blackmailed to give information about the ship so that it can be sunk.
Probably the most striking thing are the closeups of the suffering peasants.
Casablanca it isn't. Fonda and Carroll have no chemistry. The dialogue is very stilted.
Henry Fonda at the end gives an impassioned speech right into the camera. It's embarrassing.