Sunlight coming through blue tarp?

To be honest, I have never studied the process of solar radiation going through a blue tarp. But, I think I can make a fairly educated guess. So let me answer (or guess at) a few of your more pointed questions. Does the tarp block UV? Almost certainly. But UV radiation is not a major source of energy from the sun. It only constitutes a small fraction of the sun's radiation; visible and IR light constitute much more. Does it block IR? This I do not know. It depends on the material. And it depends on what spectrum of IR you are concerned with. I know plastics are not great at blocking near IR (close to visible IR), but you would think the blue dye would be fairly good at that. I would think the far IR would be easier to block. However, you should realize that the majority of the suns energy comes in visible light. Since the tarp passes a good bit of visible light, it passes a good bit of the sun's energy. And visible light can make you hot just like IR can.

So why is your hut hot? Well, three reasons. First off, the tarp probably passes enough visible and IR light to make you hot. In a confined space, where there is poor air circulation, it takes a good bit less energy to make you hot. Second, what the tarp does not pass, it will predominantly absorb. When the light gets absorbed, the tarp heats up and two things will happen. That absorbed light we be re-radiated in the from of lower energy light, that will still be energetic enough to heat you (this is called black-body radiation). The other thing is that the tarp will heat the air next to it. The heating of air will be a slower process, but will still turn off pretty quickly when the sun goes away. And finally, your body's temperature sensing ability is not based on your actual body temperature, but is in fact based on the heat transfer out of your skin. If heat is transferring into your skin, you will feel like it is hot. This radiation will largely pass through the air in your hut and be directly absorbed by your body. This gives you a lot of heat coming in quickly, but as soon as the sun gets blocked, the heat transfer slows way down. So even though the air temperature didn't change quickly, your body will sense a change quickly. Again, this is exacerbated by the low air-flow in your tent, since airflow works to slow down the heat transfer into your body by removing some of the heat in the outermost layer of your skin before it makes it to the living portion of your skin.

In the end, there are some things that will help. First, use a darker tarp that blocks more of the radiation. A reflective tarp is that much better because instead of absorbing radiation, it sends it right back out away from the hut. A double layer tarp will also do quite a bit to curb this heat (though the reflective tarp would be better). And finally, try to increase air flow. A fan would be great. But a cone shaped tarp with a hole at the top and air inlets at the bottom will also help remove the hot air and build a current that cools you.

Heat Reflective Tarp