I cannot seem to reproduce your issue. Mind adding more information or trying again? Here's what I tried out:

snapcraft-black # cat project/snap/snapcraft.yaml 
name: black
base: core18
version: git
summary: boo
description: boo
grade: devel
confinement: classic

apps:
  black:
    command: bin/black

parts:
  black:
    source: .
    plugin: python
snapcraft-black # ls prime/lib/python3.6/site-packages/
Click-7.0.dist-info	     blackd.py		   setuptools-40.8.0.dist-info
appdirs-1.4.3.dist-info      blib2to3		   toml
appdirs.py		     click		   toml-0.10.0.dist-info
attr			     easy_install.py	   toml.py
attrs-18.2.0.dist-info	     pip		   wheel
black-18.9b0-py3.6.egg-info  pip-19.0.2.dist-info  wheel-0.32.3.dist-info
black-18.9b0.dist-info	     pkg_resources
black.py		     setuptools
snapcraft-black # snap try prime --classic
black 0+git.250ba7f mounted from /root/prime
snapcraft-black # black --help
Usage: black [OPTIONS] [SRC]...

  The uncompromising code formatter.

Options:
  -l, --line-length INTEGER       How many characters per line to allow.
                                  [default: 88]
  -t, --target-version [pypy35|cpy27|cpy33|cpy34|cpy35|cpy36|cpy37|cpy38]
                                  Python versions that should be supported by
                                  Black's output. [default: per-file auto-
                                  detection]
  --py36                          Allow using Python 3.6-only syntax on all
                                  input files.  This will put trailing commas
                                  in function signatures and calls also after
                                  *args and **kwargs.  [default: per-file
                                  auto-detection]
  --pyi                           Format all input files like typing stubs
                                  regardless of file extension (useful when
                                  piping source on standard input).
  -S, --skip-string-normalization
                                  Don't normalize string quotes or prefixes.
  --check                         Don't write the files back, just return the
                                  status.  Return code 0 means nothing would
                                  change.  Return code 1 means some files
                                  would be reformatted.  Return code 123 means
                                  there was an internal error.
  --diff                          Don't write the files back, just output a
                                  diff for each file on stdout.
  --fast / --safe                 If --fast given, skip temporary sanity
                                  checks. [default: --safe]
  --include TEXT                  A regular expression that matches files and
                                  directories that should be included on
                                  recursive searches.  An empty value means
                                  all files are included regardless of the
                                  name.  Use forward slashes for directories
                                  on all platforms (Windows, too).  Exclusions
                                  are calculated first, inclusions later.
                                  [default: \.pyi?$]
  --exclude TEXT                  A regular expression that matches files and
                                  directories that should be excluded on
                                  recursive searches.  An empty value means no
                                  paths are excluded. Use forward slashes for
                                  directories on all platforms (Windows, too).
                                  Exclusions are calculated first, inclusions
                                  later.  [default: /(\.eggs|\.git|\.hg|\.mypy
                                  _cache|\.nox|\.tox|\.venv|_build|buck-
                                  out|build|dist)/]
  -q, --quiet                     Don't emit non-error messages to stderr.
                                  Errors are still emitted, silence those with
                                  2>/dev/null.
  -v, --verbose                   Also emit messages to stderr about files
                                  that were not changed or were ignored due to
                                  --exclude=.
  --version                       Show the version and exit.
  --config FILE                   Read configuration from PATH.
  -h, --help                      Show this message and exit.
snapcraft-black # which black
/snap/bin/black
snapcraft-black # snap run --shell black
snapcraft-black # $SNAP/usr/bin/python3.6
Python 3.6.7 (default, Oct 22 2018, 11:32:17) 
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import click
>>> click.__path__
['/snap/black/x4/lib/python3.6/site-packages/click']
>>>