Blanket won't help much if there isn't heat IN the tank to begin with. It will only help slow heat LOSS from the tank.
You need to be careful putting the tank in direct sun, sometimes that can actually make the tank too hot. You don't want to bake the crabs.
Do you have a temperature and humidity gauge? This is really essential equipment for a hermit crab tank. You can find or make a lot of things to use for them, but that one thing you really do need to buy. I like the digital ones that you can find at the hardware store. They cost about $10. Here are the ones I use:
https://www.lowes.com/pd/AcuRite-Digita ... 1000162483
I keep them down on the substrate (where the crabs are most likely to be), and I have at least one on each end of the tank (sometimes more, but I have a huge tank; if you only have a 10g right now you only need one, for sure). If you only have one gauge, try to put it in the coolest place in the tank, as far away from the water sources as possible. This way it will give you the lowest temp and humidity in the tank (so you can be sure you are keeping the temp and humidity at least HIGHER than that).
Once you are sure that you know what the temperature and humidity are in the tank, there are a lot of things you can do to increase the temperature to somewhere between 75 and 85 degrees Fahrenheit, which is the best range for them.
If your house is kept at or above 75 degrees in the summer, then you will not need to add any more heat until and unless your house gets cooler. I see you are in Florida, so depending on where you are in the state, you may really be okay pretty much all year around.
If it IS too cool in your tank, one way to bring up the temperature is to put a regular lamp with an old-fashioned incandescent bulb near the tank so that it shines into the tank. I actually have a small table lamp sitting on top of my 120g high tank, it's kind of like their "little sun" that I turn on during the day (I also have two UTH's on the back and one side of the tank, but the lamp acts like sunshine during the day, warming the tank up, then at night it is off, allowing the tank to cool a bit, just as temperatures normally would in the wild). Incandescent bulbs produce quite a lot of heat. A halogen lamp will also produce a lot of heat, and can be used the same way. Of course, you will want to turn the light off for nighttime, because the crabs are nocturnal and won't want bright light at night.
Again, it is really important to have a temperature and humidity gauge inside the tank before you start trying to increase the temp, because an enclosed glass box can keep in a lot of heat (think about getting into a car that has been left in the sun for a while), and while temperatures too cool can be dangerous for them, temperatures too high can also be bad. Purple pincher crabs generally want both temperature and humidity to be between 75 and 85. A little bit below that for a little while is okay, and higher than that is also okay, but that range is best for them.
I can't afford a mat or a heat lamp so I really need some tips on how to keep my crabs warm. They are not by any drafts besides my ac 
they are where the sun coming from my window hits them just right. They are on top of a blanket. But nothing is keeping them warm. Please give me some advice of how to keep my cold crabbies warm without the heat lamp or mat! Please give me tips, videos, links to websites, or anything that could help me!
