This recipe calls for 1/4 cup of gin for 6 pops, which is only 2 shots of gin total. How much more alcohol can be added to make them stronger, but...

The more alcohol you add, the harder it becomes to freeze. Since this is an adult treat (I trust), maybe consider adding the amount of booze you want and instead of making an ice-pop, make "snow-cones" or "slurpies" in shot glasses.
You can still make it two colors by doing a batch with blackberry and another with a lighter color ( like strawberry or raspberry ) and layer in the shot glass.
Also with that serving method you can garnish with a fresh blackberry, or mint leaf, or....

You should be able to put up to one shot of alcohol per cup of non-alcoholic liquid.