External flash threw off my camera focus?

I just got a new external flash (a TT560 Speedlite) for my Nikon D3100.It fires fine and everything, but as soon as I started using it, everything became blurry through the viewfinder, even when I switched it to manual focus and looked through the entire range of it. The really weird thing is, when I have it on autofocus and take a photo, the final product comes out clear and in focus--it's just that when I'm taking it I have no idea what's in focus or not. Since I vastly prefer manual controls this is a problem. Has anyone else had problems like this with external flash messing up your camera? Is there a way to fix it?

I bet if you bought a Nikon Speedlight you would not have this problem.

Neewer is not necessarily known for high quality stuff. Something is abnormal - and I hope it does not eventually damage your camera.

I'm guessing that when you installed the flash to the flash shoe on the camera, you accidentally shifted the diopter setting on the viewfinder.

I agree with Keerok. Check the diopter. It is a little lever or wheel next to the viewfinder. Just move it up and down until everything is in focus again.

The diopter is used for adjusting the focus for people who don't have good vision.