In Blend I can open the extension Gallery and see almost the same extensions as in Visual Studio. I installed the "Color Theme Editor" in Blend. It installed new Themes, like "Solarized", but the editor itself doesn't work.
I tried another approach, the Extension with Additional Color Themes was also available in Bend, so I downloaded it from here https://github.com/Microsoft/VS-ColorThemes/releases/ and added my own Color Theme. Then I compiled it and installed it. It was installed in Visual Studio but not in Blend?
I tried to understand, what went wrong, but I didn't found the problem. I assumed it has something to do with the target. But the target used was Visual Studio Shell. I assumed Blend is also using the Shell. All other targets where specific versions of Visual Studio. I didn't found Blend there.
I also didn't found anything about this subject in Google. I wonder if anyone can enlighten me a little with this?
OK, I finally was able to install a VSIX in Blend. The way to do it was via a custom Extension Gallery.
I found this page: https://devblogs.microsoft.com/visualstudio/create-a-private-gallery-for-self-hosted-visual-studio-extensions/
The author describes how to setup a custom Extension Gallery.
I just tried it with a local Feed file, with my extension as only entry (also local file). With this I was able to just install the VSIX in Blend.
PS: I only tried it in VS2019, I assume it also works in VS2017 (that my original question was for).