I'm not sure what I did wrong but your mapping didn't work for me on Neovim - it either immediately closed the terminal or didn't know what <bar> means.
All the mapping worked fine for me as well in vim, and I found gB to be super useful, earlier I had to go first in tree view and then select the file and then use the blame view.
Eventually I moved to https://github.com/codeindulgence/vim-tig which does the same via `:Tig` and `:Tig!`.
I added those mappings to achieve something similar:
Thank you for inspiration! Tig FTW!