In general, the answer is that Lava (a type of 3M Zirconia, all-ceramic porcelain crown) creates a beautiful invisible crown. In general, there is usually a nice improvement over the darkness of a PFM (porcelain fused to metal dental crown). The dark spaces in between your teeth are likely minor gum changes that will improve by itself in a few weeks.
Your dark lines could be temporary or permanent.
They could be temporary and caused by:
The gum being pushed down during the impression or fitting process.
The shape of your new Lava dental crown being slimmer than the old PFM crown or the temporary crown.
Some gum cleaning chemicals can leave a dark residue.
The dark lines could be permanent and caused by:
Internal darkness in the tooth itself, especially if it has had a root canal.
The new crown margin fitting slightly higher than the gum line, revealing the darker colored tooth
Contamination or bleeding during the cementation of your crowns.
If these are back teeth, then I would contact your cosmetic dentist and have him recheck them. Odds are high that the darkness will seem to disappear or at least improve over a few weeks. Make sure to show him exactly what you see so he can ease your mind.
Remember, these are a prosthesis, just like a glass eye. They are not real, but made to look as real as possible. Most of the time Lave dental crowns produce a really nice cosmetic result, but there are limitations, especially if this is the second time you are having these crowns made.
Like other cosmetic dentistry concerns, the key is to discuss your expectations and have clear communication. Thanks, Scott Greenhalgh, DDS.