Dilation of blood vessels will cause them to be nearer to the skin surface. This means that it will be easier for heat to be lost from the blood vessels to the skin and ultimately to the atmosphere via conduction. This loss in heat reduces the temperature of blood and hence the body temperature in general. If the rate at which blood is pumped is constant (whether blood vessels are constricted or dilated), then blood pressure decreases when blood vessels are dilated because blood pumped at constant speed has more space to pass resulting in decreased blood pressure.
