You can adjust the blackpoint with a
grade node which works similarly to Nuke’s. The slider is labelled “black” in Natron 2.3.14 and “lift” (as it is in Nuke) in 2.3.15 beta.
Keep in mind this doesn’t serve exactly the same purpose as Nuke’s
Exposure node (no colourspace option for example) but it will let you do what you want to here and likely in the future as well, I pretty much never use the
exposure node when comping in Nuke. If you want to adjust the RGB values independently you’ll need three different
grade nodes with the respective channel checked on at the top.