I also started with a dark green/black lagoon this year. As Brent stated, I knew there was a lot of debis on the bottom.

If there is a lot of debris, it won't help to start vacuuming, the filter, as well as the leaf pot basket and skimmer basket will clog in five minutes or less. I used a Polaris 65 for several days to scoop or sweep the debris from the bottom. I then started vacuuming - that took another few days. I couldn't see the bottom during this entire effort.

Part of my issue was that my ph was also low - I got it up to the proper level at about the same time as I got 99% of the crap out and pretty much overnight my pool cleared up.

Not getting as much as that stuff out as possible will indeed prolong the process and you'll end up with a lot of belach bottles in your backyard.