When building the documentation, either manually or via dart client, the error message "'<PATH>/BlueBerry.tag' does not exist or is not a file" is shown. This does not prevent the documentation from generating, it just removes links to the BlueBerry documentation.

The reason for this seems to be, that the BlueBerryDoc target does not build. Instead you get the following error message:
CUSTOMBUILD : error : tag OUTPUT_DIRECTORY: Output directory `/MITK-build/BlueBerry/Documentation/reference/api' does not exist and cannot be created
which aborts the creation of BlueBerryDoc.

/MITK-build/BlueBerry/Documentation/ does exist /MITK-build/BlueBerry/Documentation/reference does not.

I created the subdirectories on my dart client and the BlueBerryDoc build worked and the tag file was created.

One possible solution in my opinion would be to just generate the subdirectories via cmake.