Isabel is lavender diluted buff. Porcelain should have more lavender in the mix, showing both colours equally (look at a photo of a porcelain d'uccle), and you should really have mottling.
As to light versus dark blues, it depends on the parents; if hte parents are lighter in shade, the babies are more likely to be lighter. If the parents are darker, the babies are mre likely to be darker. But in either case there is likely to be some variability from one bird to another.
Dark blues can look black, especially in artificial or poor ligting. The blackest blacks are very obviously black, however.