what tom said!! I agree with it all. I will just elaborate on the "swimmingly wet." Your bonsai is in soil that is all wrong for it. (It may be the soil it came with, bonsai trees are frequently sold in very bad soil - guarantees repeat customers I guess
![Sad :(](./images/smilies/icon_sad.gif)
).
Real bonsai soil is very mineral, loose and free draining. What you have is peat moss and dirt. Holds a lot of water when wet, stays wet for awhile, but then dries out and gets water repellent. So your soil is too heavy, excludes air, holds too much water, too long. Here's a lot of information about bonsai soil with links to more:
https://www.helpfulgardener.com/forum/vi ... =36&t=3422
I would summarize the soil info by saying that bonsai soil would be mostly pine bark and mineral grit (fired clay, granite grit, ground volcanic rock, etc), with maybe a little bit of humus.
You didn't say anything about how/ how much/ how often you have been watering it. Never water bonsai on a schedule. Water when they need water, which will vary with temperature, humidity, season, how fast the tree is growing, etc.
Here's some good information about bonsai care including watering and how to know when to water:
https://www.helpfulgardener.com/forum/vi ... =36&t=1479
Yours has clearly been staying too wet. The black area at the base may mean it is starting to rot. You need to get it in real bonsai soil. When you take it out to repot, gently wash all the old soil off and check the roots. If they are black, mushy, and or smell bad, the roots rotted. At that point, you might as well pitch it and start over, after you have done enough reading to know a little bit more about what you are doing the next time. If the roots are firm and white and healthy, repot your little tree in good bonsai soil.
Best wishes! Keep us posted on what you find and how your tree fares....