The negative electrode of the car generator is designed to use the metal of the car body to form a path, commonly known as "grounding".
When you touch the ground wire with the positive pole of the generator, it is equivalent to a short circuit between the positive and negative poles of the power supply, resulting in a huge short circuit current. This will blow the fuse. It's a kind of protection. So if you touch the positive pole to the ground, you won't see any sparks.
You can check the fuse box to see which fuse is blown and replace it with a new one.