Selur's Little Message Board

What is the best way to adjust only the black levels? I tried with the "Levels" filter in Vapoursynth, but it seemed to make the image more or less greener.

No, clue what you want to do exactly, so there's what usually comes to mind (assuming your source if flagged properly):

• use Levels with 'luma only' (or with 'Use RGB' depending on your preference)
  
• use a color matrix or LUT
  
• use Smooth Levels
  
• use Limiter if you want to 'cut off' a range
  

Depending on your source, stuff like 'Auto white', 'Retinex' or 'Grayworld' might also work.

Cu Selur

Ps.: from the green part I guess that you only want to adjust the luma and not all channels.