I want to take portrait photos with blurred background using Nikon D3100 18-55 lens

Blurring the background requires a large aperture and long focal length in terms of hardware conditions. The maximum aperture of the kit lens at the telephoto end is only F5.6, which is obviously not wide enough. You can only keep the subject away from the background and as close to the camera as possible, so that the "mug photo" taken will have some blurring effects. If the subject is further away from the camera, it will be difficult to obtain an obvious blur effect even in a half-length photo.

As for the overexposure problem you mentioned, it should not exist. Although it is a large aperture, the camera will automatically increase the shutter speed after metering, which generally does not cause overexposure. Even if it is overexposed, you can also reduce the exposure compensation to solve it. But under the settings you mentioned, the blur effect is basically impossible to obtain.