I like to learn things even outside my university degree and I wonder if it is moral to read articles found online on blogs and other sources?
There's nothing morally wrong with reading blogs and most online sources, so I'm going to assume you mean published articles and books.
When it comes to journal articles and books, it depends. Many articles that are freely available online are legally published / published with the author's permission. Others are not. Obviously papers posted on the author's website, or websites like arXiv are okay, but for other listings it's a lot less clear. It's up to you to make your best judgement as to the legality and the morality of the access you have.
This also of course presumes that you think it's moral to support closed-access journals. Some people feel that closed-access journals are a bad thing and should be opposed. To those people, doing anything other than seeking out illicit versions would be morally wrong.
Note that many journals allow authors to post preprints or even the "official" full text on their websites. In such cases, there's clearly no ethical dilemma involved.
In cases where such behavior is prohibited by the journal, then there is a bit of a dilemma. The "fault" is shared, between the person who posted the infringing content and those who read it.
If you're talking about research papers, there is absolutely no ethical reasons why you should buy the access to them. In fact, publishers like Elsevier are taking advantage of the power they have to gain money on what's free and open : Knowledge. Research papers are never meant to be sold but shared so the unethical part comes from publisher. If you want to find free research paper, go on Sci-Hub!
There is the concept of open access wich many journals are starting to focus on more. There are many quality peer-reviewed papers that are open access. I think most easy way to reach those quality papers is through scholar.google.com. As far As I know when you see the PDF or HTML link in the scholar.google.com, they are open access and you can find most of the paper's abstruct there even if they are not available. If you are interested on those, you can mail to author as said above. In short, If you find open access journals, you can read them, to find legal, open access articles, you need to choose your websites carefully or you can ask for full text articles that you can't find online.
Reading things on blogs is fine, it's what blogs are for, but you shouldn't cite them, blogs are not peer reviewed. Well, maybe you can cite them as "personal communication", obviously you shouldn't plagiarise them.
There are a number of legitimate ways to get PDFs on line, for example if they were published open access (the journal or conference lets you have it for free, probably because they charged the authors or their universities already), or if the authors put a pre print online (this is entirely legal) or if the authors or someone else put the final version illegally on line.
This last, being illegal, is as such at least somewhat immoral; it is theft from those who hold the copyright. The next question is, are you culpable for reading such a PDF, or indeed obliged to figure out whether a PDF you find online was "liberated" illegally, or whether it was circulated legally? Even if you decide or know that the PDF is illegal, if you are a utilitarian of some form you might ask: is the good that would come from you reading it greater than the (probably negligible) harm you do by reading it? Of course, other forms of ethics never accept that the ends justify the means.
Thrown into the mix, it has long been legal and common practice that if you ASK an author for a copy of their paper, they will just send you an offprint. In recent decades, this is a PDF. So you might also consider starting to read a paper, deciding whether its good, and then asking for a legitimate copy that way. Or you could decide the good you would do by asking he author is outweighed by the time you would cost the author, and keep on reading the PDF you already have.
As an academic, if I find out about a paper that I can't read, I both ask the author for a copy AND ask my library to buy access to the journal. That's because I think it's far more moral for publishers to charge for reading than for authoring an article, but I also think the reason I accept a low university salary is to be in a university with access to knowledge. That's another way to read articles legitimately of course – go to a library!