I got my first BBL treatment 3 weeks ago by a plastic surgeon. How fast do dark spots darken after a BBL treatment?

The first BBL treatment always seems to give more noticeable results.  The easy pigment has been treated with the 1st session and the pigment left for your 2nd and 3rd treatments is harder to remove.  This is why we always recommend a series of BBL treatments.
Also, if your laser tech was targeting the redness - the settings are different and the pigment may not be treated as well.
I have seen sunspots darken when the patient had sun exposure after the treatment - this is normal.  That why we recommend sun avoidance after BBL treatments and that patients always need maintenance BBL treatments - because we all can't live in a cave!
I would recommend going back for a third treatment and telling your laser tech you want to focus on the sun spots and that you didn't see a big response last time - maybe they will increase the settings. A visit skin evaluation may be helpful for you to see the ongoing improvements.
Ken Oleszek, MDLa Fontaine AestheticsDenver, Colorado

Thank you for your question.  Every patient is a little different - in many cases, patients notice their freckles and dark spots darken immediately after treatment, while in other cases it can take a few days.  Generally speaking, it can take up to 1-2 weeks to see the effects of a BBL treatment on the face.  I would recommend following-up with your plastic surgeon at that time to review your before and after photos and discuss your concerns.  I hope this helps!

Some patents immediately, others notice the MILO effect within 4-6 days. All heal within 7-9 days, one can accelerate healing with 1. Use of emollient frequently 2 Gentle gauze pad or Buff Buff pad to exfoliate at day 5-8. Be gentle. All the best, Dr Davin S. Lim 

Yes, some patients experience immediate darkening of these spots while other patients will see darkening over a week following the treatment. 
BBL treatment is a spectrum of treatments. Each visit may concentrate on different problems (browns, broken capillaries/veins, texture) depending on your concerns. The provider will chose appropriate settings to address those concerns.There could be a gradual increase in the strength of the treatment should your skin response level out. Keep in mind that sometimes the effects of the laser may not happen on a surface level visible to the naked eye. I recommend you have your Doctor use a Visia Skin Analysis in order for you to better see all the improvements both on the surface and subsurface of the skin. Make sure you discuss with provider the concerns and goals you have for your treatments so they better address them. I hope this is helpful!