Hi Nicole,
Just to mention first off my comment's more regarding sub domains more than Hubspot specifically. I'll try to answer your question directly:
Does one lose any credibility or get diluted by putting said blog on a subdomain rather than in a subdirectory folder?
You may loose credibility (or gain if you put it on a sub folder) by the fact a subdomain could look 'new' compared to an existing website but to be honest its nominal in the big picture. As for credibility that's more for the users point of view than Google's (unless you're planning on being very naughty with the site) but with good content and branding this shouldn't be a problem.
There are oodles of topics on subdomains vs. subfolders out there for you to research but try not to over think it too much as it really all boils down to what you want out of it. You can over come most of the problems you feel you may have. Link diluting etc. is only really a problem if you're planning on posting the same content on both domains as they would literally competing with one another but if its unique and useful info they it should rank on its own merit (with some basic SEO/SEM)
If you are unsure or feel inexperienced I'd recommend putting it on your site then you will get a boost from your blog (see Moz example above) without as much work, so unless you want to distance your main site from the blog this should cover you well.
if in doubt do more research, ask more people get all the info until you feel you're all covered.
Best of luck with your blog and I hope it all goes swimmingly.