BUG: Fixed block placment from reindex(?)
Closes https://github.com/pandas-dev/pandas/issues/24147
Hello @TomAugspurger! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/internals/blocks.py`](https://github.com/TomAugspurger/pandas/blob/3be6e0dd9e848c90306e82f7333947efb4aa472f/pandas/core/internals/blocks.py) !

 - There are no PEP8 issues in the file [`pandas/tests/extension/base/methods.py`](https://github.com/TomAugspurger/pandas/blob/3be6e0dd9e848c90306e82f7333947efb4aa472f/pandas/tests/extension/base/methods.py) !

 - There are no PEP8 issues in the file [`pandas/tests/internals/test_internals.py`](https://github.com/TomAugspurger/pandas/blob/3be6e0dd9e848c90306e82f7333947efb4aa472f/pandas/tests/internals/test_internals.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=h1) Report
> Merging [#24149](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f492be63d7e904c2e0fa8113a72b761ee4b74219?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24149/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24149      +/-   ##
==========================================
- Coverage    92.2%    92.2%   -0.01%     
==========================================
  Files         162      162              
  Lines       51700    51700              
==========================================
- Hits        47670    47669       -1     
- Misses       4030     4031       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <100%> (-0.01%)` | :arrow_down: |
| #single | `43.02% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/24149/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `93.65% <100%> (-0.07%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=footer). Last update [f492be6...3be6e0d](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=h1) Report
> Merging [#24149](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f492be63d7e904c2e0fa8113a72b761ee4b74219?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24149/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24149      +/-   ##
==========================================
- Coverage    92.2%    92.2%   -0.01%     
==========================================
  Files         162      162              
  Lines       51700    51700              
==========================================
- Hits        47670    47669       -1     
- Misses       4030     4031       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <100%> (-0.01%)` | :arrow_down: |
| #single | `43.02% <100%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/24149/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `93.65% <100%> (-0.07%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=footer). Last update [f492be6...3be6e0d](https://codecov.io/gh/pandas-dev/pandas/pull/24149?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Going to merge this now, and merge https://github.com/pandas-dev/pandas/pull/24114/files on top of it, which will enable the rest of the tests. I'll fix the stale comment in 
https://github.com/pandas-dev/pandas/pull/24149/files/3be6e0dd9e848c90306e82f7333947efb4aa472f#diff-8f51ce8697bdff680b3f71b196c582ee as well.