Strawberries start green, and then gradually turn white before protein causes it to turn the familiar red color, indicating that the strawberry is ready to be eaten. Red is a primary color, as is the color blue, so making a red strawberry turn blue is impossible unless you coat the strawberry in something blue. However, if you use a variety of strawberry that is naturally white, you can make it change color easily.
Remove the green central core from the white strawberry using a small knife; you may be able to pull it out using your fingers if the strawberry is very ripe. This tears the cells in the strawberry.
Fill a small jar with cold water. Add a few drops of blue food coloring. Mix the water and food coloring using a teaspoon. If you want a deeper blue strawberry, then add a few more drops of coloring until you get the desired color.
Put the strawberry in the colored water. Put the jar in a place that has plenty of light, but not direct sunlight; you don't want the water to get warm; otherwise, the strawberry will soften too much.
Leave the strawberry in the jar for a day. It will gradually absorb the blue color from the water. The more coloring you put into the jar, the quicker this will take place.
Remove the strawberry from the jar after a day. Check that it's the blue color you want. If you want it a darker blue, then leave it for about 12 hours and check again. Don't leave the strawberry for more than two days, as it may turn mushy.
Carefully pat the strawberry dry using a paper towel when it's reached the color you want. Eat the strawberry within a few hours.