Nesting Blackbird

So the other day I was delighted to find a blackbird had built a nest in a plant next to my lounge window and then yesterday had laid at least two eggs that I could see. She left the nest yesterday evening and I didn't see her return until this morning so she was absent for around 11 hours. She's now sitting quite happily again and I wondered if this is common and she would be laying additional eggs to brood, if the two I had seen were likely to be unfertilised or just won't hatch because they were left? Any ideas would be appreciated!