Blood doesn’t thicken in winter, even though it may sometimes feel like it.
In fact, it might tend to be thicker in summer, when the body loses water through sweating and people need to drink more to remain hydrated.
Temperature is not the chief regulator of the thickness of blood.
It depends on the kidneys, which sense blood volume, and the brain, which senses osmotic pressure, or the concentration of chemicals in the blood.
The kidneys regulate how much water the body excretes as urine, and the brain may signal a need for more water by making the person thirsty.
Hormonal signals are also involved.
As for being able to stand the heat or the cold, which is what many people mean by thick or thin blood, the blood doesn’t have as much to do with it as the very complex relationship between glands and the thermal controls in the brain.