The repeal of "Don't ask, don't tell" had no effect on whether gay military personnel could be open.
Attitudes towards gay men in particular in the forces didn't change just because people could legally be "out".
The Prussian blue test for cyanide doesn't actually use Prussian blue, it creates it.
Iron sulfate is added to the suspect compound or solution, and if cyanide is present, Prussian blue is produced.