Hello!
As of my last knowledge update in September 2021, the Nintendo DSi doesn't have built-in blue light filter settings like some modern devices do. The DSi was released before the widespread awareness of blue light's potential effects on sleep and eye strain. Therefore, it might not have such features integrated.
If you're looking to reduce blue light exposure while playing on your DSi, you might consider using external solutions such as clip-on screen protectors with blue light filtering capabilities. These accessories can help reduce blue light emissions and potentially improve your comfort during extended play sessions.
Keep in mind that my information might not be up-to-date, so I recommend checking more recent sources or Nintendo DSi communities for any developments or third-party solutions that have emerged since then.