Wash the grapes and add them to a blender. Blend for a very short time, about 5 seconds, until you get a coarse purée.
Add the blended grapes and sugar to a heavy-bottomed pot. Cook on a medium-high heat, uncovered, until the juices start to thicken. This should take about 15-20 minutes. Stir occasionally, skimming off any foam that rises to the surface.
Test: scoop a ladle of the jam and pour it back slowly into the pot. You want it to be syrupy and slightly viscous, not a loose runny liquid. If it’s too runny, continue to cook for a few more minutes until you get the right consistency. (Note: it will thicken slightly once cooled and also when it's refrigerated.)
Strain the jam if you like it smooth. Ladle into jars and refrigerate once cooled. Stays good refrigerated for up to 6 weeks.
The amount of sugar you'd need to add varies vastly on the sweetness of your grapes. Mine were very sweet so I could get away with 1/4 cup. Taste, add more, and cook it out for a bit longer if yours are more tart.