We also often close the blinds on the ground in the Summer or at hot destinations to help keep the aircraft cool.

You can choose to have your blind up or down unless sitting at an emergency exit row, where it needs to be up. Some countrie's regulatory authorities do have different rules but in Europe, EASA, does not stipulate either up or down unless at an exit row.

Some airlines may well adopt their own practices also but at BA we leave it to the customer to choose.