If the fuse blows when you turn on the pump switch, then you might suspect a short. If it blows when demand causes the pump to try to start pumping, then the problem is most likely the pump. Feel the pump body and see if it seems hot.
The fact that it is the fuse in the panel that blows rather than the one by the pump could have several possible explanations. Are both fuses the same amp rating? If not, the lower rated fuse will be the one that blows. Are both fuses the same type? Different types of fuses have different time/current curves. With a given amount of overload, one type can blow significantly faster than some other type. Some are even designated slo-blow.
One other thing to check is if the fuse that blows has other loads connected to it. You know that the one at the pump does not.
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