Q: Some "blue lines" show on my mac's display when the display is showing colour close to black.

Hello Zhonghao Xia,

After reviewing your post, it sounds like the internal display on the MacBook shows some lines on it. I would recommend that you read this article, it may be able to help the issue.

Apple computers: Troubleshooting issues with video on internal or external displays - Apple Support

You can reset the Mac's parameter RAM and SMC.

Thanks for using Apple Support Communities.
Have a nice day