After long exposure to light - screen light in your case; the receptors or sensors in the eye get exhausted and take time to recover. During this phase ; a transient blackout is normal and not very worrisome. You can use frequent breaks to avoid the same and proper posture while working on screen. Also, it adds to your comfort if you use lubricating eye drops like - COMFORTZ EYE DROP 3 times daily.
Next Steps
Take frequent breaks and add eye drops as suggested.
Health Tips
Visit Ophthalmologist SOS