Black: ModuleNotFoundError: No module named 'black'

[aranmohyeddin]

Operating system: Archlinux
Python version: 3.7.1
Black version: 18.9b0

I have followed the guide and I use vim-Plug I have successfully installed the plugin and it works just fine.
then I do workon mypersonalvenv and after that if I wanna open vim it says:

$ vim main.py 
Error detected while processing /home/aran/.vim/plugged/black/plugin/black.vim:
line  124:
Traceback (most recent call last):
  File "<string>", line 56, in <module>
ModuleNotFoundError: No module named 'black'
Press ENTER or type command to continue

when I press enter and then run :Black I get:

Traceback (most recent call last):
File "", line 1, in
NameError: name 'Black' is not defined

when I run :PlugInstall it says:
- black: Already installed
Also vim has created these files:

/home/aran/.vim/black
├── bin
│   ├── activate
│   ├── activate.csh
│   ├── activate.fish
│   ├── python
│   └── python3
├── include
├── lib
│   └── python3.7
│       └── site-packages
├── lib64 -> lib
└── pyvenv.cfg

6 directories, 6 files

[aranmohyeddin]

The problem was that I was already inside my personal environment when I ran :PlugInstall now I removed ~/.vim/black and deactivaded my venv and opened vim again(automatically installs black) and then did workon mypersonalvenv and it works just fine now.