Put water and sugar in a saucepan. Bring to a boil over medium-high heat, whisking until the sugar dissolves. Simmer for 10 minutes. Remove from heat, add the mint leaves, cover and let steep for one hour. Pour through a strainer into an airtight glass container. Refrigerate until ready to use.
To prepare the mojitos:
In a large pitcher or drink dispenser, combine the dark rum, blood orange juice and mint simple syrup. Mix well to combine.
Add 1 - 2 fresh mint leaves to the bottom of each 16 oz glass.
Fill each glass half full with the rum mixture. Add about 1/2 C club soda and mix well.
Fill each glass with crushed ice, give them one more stir and garnish with a slice of blood orange and a sprig of mint.