I noticed this blue dot and found out it is known as a stuck pixel. I found a white one in another image - a panning shot. In this image attached there are a couple more blue stuck pixels.
I can't tell if they are hot or stuck pixels (see: Hot, stuck, or dead pixels. What's the difference?). Stuck and dead pixels are completely normal, they happen. As a matter of fact, there are almost certainly more stuck pixels in your camera than you even know about. The camera maintains a "pixel map" of known hot pixels, and automatically filters them out before the sensor data is ever written to storage cards, even in raw files (
.nef, in the case of Nikons).
Also, tools such as Lightroom and Photoshop / Adobe Camera Raw will automatically eliminate hot, stuck, and dead pixels when importing raw files. So if you shoot raw (and I generally highly suggest you do), check if your post-processing software will handle them for you so you never have to worry about it.
You can make the camera update its pixel map by performing two back-to-back sensor cleaning cycles — "Clean image sensor" in the Setup menu. It will find stuck and dead pixels, and save the pixel remapping.
To reduce hot pixels in long-exposure photographs, you can enable "Long exposure noise reduction" (LENR), which will take your photo, then immediately take a "dark frame" image of the same duration. It will then subtract the dark frame (which is just averaged noise + hot pixels) from the captured long exposure image, and produce a single photo. The downside of LENR is that it takes twice as long as your shutter speed to take the image. So a 30-second long-exposure shot will actually take 60 seconds to capture and process, before the camera is ready to take another shot.