Why is my toe nail is blue? My big toe nail turned blue for no reason, it looks bruised but I never dropped any thing on it. It looks like a blood cloth under my nail. This has happned to my sister and her toe nail fell off. Maybe we share the same shoes

Bleeding under the nail. The most common reason why a nail become the blue is usually due to blood but that accumulates under the nail. This could be due to direct trauma or it could be due to shoes that are too short and there is continual trauma to the nail and eventually it will turn blue.
Black toenail . With no injury history it is advisable to have this evaluated by a physician to ensure it isn't more than just a subungual hematoma (blood under nail).
THANK . Thank you for providing a photograph! when dealing with such issues, it's tremendously helpful. If your toenail turned blue suddenly (not slowly over the course of weeks or months), based on the picture, i would agree with you that it is blood trapped under the nail plate. Our nails pick up their color from the underlying skin. If you squeeze the end of your finger to blanch the skin white, you'll also notice the nail appears white as well because the color of the nail is reflective of the color of whatever is underneath the nail. What probably happened was you broke a tiny little blood vessel and the blood had nowhere to go, so it accumulated under the nail plate and that's where it will stay until it slowly grows out with the nail over time. If this is the first time something like this has happened, i wouldn't be alarmed or pursue it unless it becomes painful or doesn't grow out with the nail over time. If this is a recurrent phenomenon, it would be prudent to determine just why your capillaries under the nail are so fragile as to break (unless you can recall some sort of trauma). You're allowed this one time event without further work-up, though. If you decide to paint your nails to cover it up (which is ok to do), it's important to remove the nail polish every couple of weeks and see what's going on down there. Hope this helps!