This is a perfect example of soil fungi taking care of the problem while you worry about what to "do". You really don't need to "do" anything if the soil fungi are healthy and working.
Does this area smell bad? I don't mean just odors wafting up from the septic. I mean dig down two inches, get a handful of dirt and smell it. If it smells like sulfurous swampy marsh dirt, then you still have a problem. If it smells earthy and has an odor reminiscent of
mushrooms, then all is well.
Do mushrooms pop up in this area after a heavy rain? If so, that is another indication all is well.
I wouldn't be worried about the hugelbed with the blueberries, the
wood you buried
should be teeming with fungi that are eating up septic tank escapee microbes. If you feel you need to do something, spread around a couple inches of moldy wood chips. All that extra fungi will quickly clean up any problem that remains.