Good idea, I don't think there would be any free course that gives certification on Blockchain. Coursera and Udemy and others probably have some good courses on Blockchain. There are somethings you have to consider like, you have to have a specific focus on the course you want because you can't just go learning all once and don't forget Blockchain are built with different mechanisms like Bitcoin, it is using the PoW while some PoS and others. Aside this, are you planning on becoming a Blockchain developer, technical writer on Blockchain or others this are things to be considered (though they are others) in order to get a course that suits you interest.
If are planning on sticking with learning about Bitcoin, this forum will be your best guide in case you ever have an issue, more like a term you understand or others during your research. Here are some good resources:
Mastering Bitcoin bookLearn me a BitcoinYouTube , This channel teaches about hardware and software relating to bitcoin.
Also I see adding the fact that you are Bitcointalk member to your CV would actually increase your stance of getting a job and others.