GPL FAQ: https://www.gnu.org/licenses/gpl-faq.html#NonFreeTools
In the old days proprietary compilers was the norm. If “blue” is of value an open source equivalent will be made eventually. But looking at the blue examples and sdesk repo I doubt it.
Going just by the examples, Blue itself seems more an incomplete templating/code generation layer for getting some syntax sugar than anything else. Like you write Blue targeting C, write super high level constructs in Blue, then include C headers and snippets of C code for all the stuff you can’t write in Blue, and finally transpile Blue into C which is then compiled conventionally.
I’d usually do the former because by build number I usually mean pipeline or job id in a build server. You could build 4.0.4 and then 3.4.18 and so 4.0.4 could be build number 1026 while 3.4.18 is 1027.
You can also just use a special number to keep your version number unique when doing dev builds so your version number comes through like 3.5.2-48 and some might call the 48 a build number, in which case that would make sense to reset with each version number.