Is blood that decreases in viscosity as it increases in temperature plausible? It's supposed to be a biomodification that decreases the amount of energy the heart needs to put in during strenuous physical activity; I'm just not sure what biochemical mechanisms there might be IRL that would enable such a thing to exist.
That the viscosity of a liquid decreases with increasing temperature it is very possible. Wikipedia has a section dedicated to the various possible models and relations to calculate the change based on the temperature.
However I think you are looking at it from the wrong side. The core of a living body is usually at a constant temperature, while the extremities fluctuate more, usually being at a lower temperature than the core. This would mean that the viscosity would increase, which, topped with the reduced section of the blood vessels at the far end of the body, would result in an increased effort for the heart.
If any, you want your blood to not vary much its viscosity with temperature. And don't forget that a couple degrees is already a large variation in temperature for a living being.
Maybe, but it seems implausible.
Fats and oils can have these sorts of properties, where viscosity decreases as temperature increases. However, they tends to have a melting point slightly below human body temperature, and as a result, if the body temperature dipped slightly below that, the organism would face the risk of their blood congealing into a solid mass.
Additionally, different sorts of chemicals dissolve in oils when compared to those that dissolve in water, so the being in question would have to have an entirely different oil-based biochemistry, rather than a water-based one.
This is a "just in case" comment, meant to cover a possible scenario where viscosity isn't technically what you were looking for, as this is a common misconception outside health sciences. Of course you as an author decide what crazy worlds we would explore, but I'd just like to note that many languages talk casually about "blood thinner" medications and use terminology which only superficially seems to overlap with the idea of physical viscosity. Such medications and physiological events nonetheless are about a biochemical reactions which affect the process of blood coagulation due to complex interactions between many proteins and other factors – it's a not a simple temperature-dependent change in viscosity. The extreme case is clotting, which can look pretty terrifying.
Hopefully this is actually a good treatise of physical (non-chemical) properties of blood, though. d(^_^)