Can bonding fix black triangles between my porcelain veneers?

Unfortunately, it's not recommended in such cases. The resin bonding to ceramic is still a challenge and the success rate here is low. Secondly, the gaps in your case are very small and close to the gum, which make the repair with composite resin not suitable solution.

Unfortunately, bonding to porcelain is not a predictable process and isn't done in this scenario. In your type of situation, I've actually used fillers like Juvederm in your gum tissue and that can plump it enough to block the space. It's not a permanent fix, but can work for a period of time. 

Unfortunately, dental bonding (i.e. white fillings or composite fillings) will not last long adhering to porcelain. A sure way to address those “black triangles” is to purchase new porcelain veneers or now there’s another great alternative solution! We now use hyaluronic acid dermal fillers such as Juvéderm Ultra XC to lengthen the gum tissue to close up those dark spaces along the gum line between the teeth. Filler lasts up to 6-8 months so it can be a short-term solution. We simply inject small amount of filler material into the papilla carefully to close up that “dark triangle”. Again, go to an experienced cosmetic dentist that is experienced in dermal filler injections. He or she need to have an artistic eye to make sure when the filler material is injected that a natural sharp points are recreated versus big round bulbs of gum tissue between your teeth.