1/ It could be that you have got damp/wet in the blower fan from leaves blocking the drain holes in the scuttle near the wiper spindle area. I would remove the pollen filter and check for damp/wet in there and dry it out if there is any and make sure of no leaves blocking the scuttle.
2/ the motor comutator or brushes could be dirty or contaminated and causing sparks between com' and brush. If I've lost you then go take a look in through the air vent in the side of a mains powered drill as it's running and it will become clear. These sparks can get quite fierce and cause the motor to pull more current than the fuse is rated at.
3/ Either that or as said the motor could be a bit tight and this will cause it to get warm/hot and then pull more than the fuse can handle. fitting a higher rated fuse will mean that the motor will get hotter before it blows the fuse.
As you can imagine both no2 and no3 potential causes involve sparks and or heat so it's in your interest to repair it rather than cause a fire.
I would deffo pop the blower motor out and clean/
service it as it costs nothing other than an hour or 2 of your time. If the blower spins up free and easy by hand you can forget no3 and look into no2.