Both of the stories I will be looking at from Blackhawk 123 (April 1958) are by Dillin and Cuidera.
The first story in the issue brings back Killer Shark, and have him kidnap a scientist who has developed a magnet that draws gold out of seawater.
Since the Blackhawks have no idea where to hunt for Killer Shark, but know about the scientist's discovery, they paint a submarine with gold, and then allow the magnet to bring them to Killer Shark. Smart.
The plan works, and the Blackhawks are able to free the scientist and destroy the magnet, although Killer Shark gets away, as he always does.
Killer Shark returns a few months down the road.
The cover story takes Blackhawk to an alien world. Martians capture the team, and bring them to a tiny planet they are using as their base for the invasion. They intend to make Blackhawk Island their beachhead on Earth.
The team get rescued by a nice Martian, who even shows them a cache of Martian weapons. The Blackhawks use the Martians own weapons to drive them off, and retake Blackhawk Island.