Works here:-

 sudo pip3 install adafruit-blinka
Looking in indexes: https://pypi.org/simple, https://www.piwheels.org/simple
Collecting adafruit-blinka
  Downloading https://www.piwheels.org/simple/adafruit-blinka/Adafruit_Blinka-2.2.0-py3-none-any.whl (79kB)
    100% |################################| 81kB 123kB/s 

Works here:-

 sudo pip3 install adafruit-blinka
Looking in indexes: https://pypi.org/simple, https://www.piwheels.org/simple
Collecting adafruit-blinka
  Downloading https://www.piwheels.org/simple/adafruit-blinka/Adafruit_Blinka-2.2.0-py3-none-any.whl (79kB)
    100% |################################| 81kB 123kB/s 

..

- Original Installation
Raspberry Pi reference 2019-06-20
- Current OS
Distributor ID: Raspbian
Description:    Raspbian GNU/Linux 10 (buster)
Release:    10
Codename:   buster
- Kernel
4.19.58-v7l+
- Model
Raspberry Pi 4 Model B Rev 1.1