Can you send a link to the page on Blurb's site that says that Lightroom is free? That is definitely incorrect. There is a subscription based plan for $120 per year ) +/- depending on where you live for taxes.
Lightroom should be able to create and upload books to Blurb without any subscription. If you have 6, it should work exactly as always. With the subscription version, the Book module should work even if you are not paying.
Do you have Lightroom 6?
Blurb does make it appear that Lightroom & InDesign Plug-in are free but I believe in the statement at the top of the page "Get Started With Our Free Tools", they are just referring to Blurb owned software on the same page i.e Bookwright, their own iOS App etc.
The InDesign Plugin is free but it requires that you already own or that you separately purchase InDesign to use it.
When you click on the 'Learn More' link for Lightroom at the bottom of that page it says:
"Please note: Use of the Blurb book module requires purchasing Lightroom software directly from Adobe."
Also they currently offering a 15% discount to Creative Cloud (this is for the full Adobe suite and it may be more than you need?)
If you just need Lightroom you could get the Adobe Photography plan bundle for about 1/4 that price ($9.99/month vs $45/mth for a full CC package.) Check Adobe site for more details. (Or purchase LR6, note it is 'End of Life' and no longer gets any updates but can be still found online)
Note: Blurb works with Lightroom Classic/ LR6 and not the new Lightroom CC cloud version