blt software package provides graphics extension library for Tcl/Tk - run-time, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install blt
blt is installed in your system.
Make ensure the blt package were installed using the commands given below,
$ sudo dpkg-query -l | grep blt *
You will get with blt package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Libraries |
| Package | blt |
| Version | 2.5.3+dfsg-3 |
| Description | graphics extension library for Tcl/Tk - run-time |
| Sub Section | Libs |