BUG: to_dict_of_blocks failing to invalidate item_cache
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

doesn't this close an issue? > doesn't this close an issue?

Not that I know of.  to_dict_of_blocks is only used on one place outside of `pandas._testing`, and that was just made removable in #35861 was thinking of https://github.com/pandas-dev/pandas/issues/35739 (but likely this is comething else)