Combine your frozen watermelon cubes and fresh lime juice in a high-powered blender. Blend until the mixture is completely smooth and well combined. If you like a little heat, feel free to add 1-2 small slices of fresh jalapeño at this point.
Pour this smooth watermelon base into your Ninja Creami pint container, filling it up to the maximum fill line. Place the pint in the freezer and let it freeze solid for at least 24 hours. This is key for the perfect texture!
Once your pint is frozen solid, remove it from the freezer. Briefly run the pint under hot water for about 60 seconds to slightly loosen the frozen block, then install it into your Ninja Creami machine. Select the "Sorbet" setting for the initial spin.
After the first spin, carefully remove the outer bowl lid. Now, add the Blanco tequila, triple sec, and your honey (or preferred sweetener) directly into the pint. Reinstall the lid and press the "Respin" button to mix everything beautifully.
Check your frozen margarita. It should be wonderfully creamy and have a smooth, slushy texture. If it's still a bit crumbly or not as smooth as you desire, add 1 tablespoon of water and spin again on the "Respin" setting until it reaches your ideal consistency.
To prepare your serving glasses, moisten the rim with a lime wedge and then dip it into Tajin for a zesty, spicy finish (if you like!). Pour your creamy watermelon margarita into the prepared glasses. Garnish with a fresh lime wedge and a slice of jalapeño for that extra flair.
Serve immediately and savor this deliciously refreshing, healthy-ish frozen summer cocktail!