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