They can. Most of my experience is with colubrids, so I don't know how likely BPs are to lay unfertilized eggs. In my experience a female of breeding age is more likely to have an unfertilized clutch if there are males around and she's gearing up to breed. I had a problem with one of my girls retaining unfertilized ovum last year that required surgery, and the vet said that the unfertilized ones are more likely to cause problems than fertilized in both constrictors and colubrids.