Adding Blog to previouis custom site build

SO I am working with a client and using a custom build that they had done for their website. In trying to go through the normal steps to build a basic blog for them the functionality did not seem to be there. So I contacted the original site builder who informed me that there had been no “blog functionality” or at least blog section built into the sites core build.

Is there a way to build in a blog section into a custom build? I can’t really throw the URL up here at the moment, but has anyone ever come across this?

I would love any advice!!!