Blankets in summer? The only blanket I have ever seen on in summer is a fly sheet.
Here it rains a lot in winter, cold miserable rain, just short of freezing. And windy.
But the horses know to go under the trees. Also, if you don't brush the long winter hair when it's wet, the water will roll off the surface "guard" hairs. YOu will see the kind of pointed "V"s that show how the water drips off their backs , keeping the under hair perfectly dry. If you try to tidy them up by brushing out that "v's" pattern when they are wet, you will ruin their duck coats. The water barrier will be broken and it will now go down to their skin and make them feel the cold as it evaporates.
If they are wet on top, dont' brush them. Don't put a blanket on top. Let them be.
They can even have snow on their legs, in clumps stuck to the fetlocks, and even on their rumps, but it does not melt becuase the insulation barrier is THAT good. Let it be.
(didn't someone get famous with that line? "Let it Be")