blogs, tumblr profiles, subreddits

Those will be the least reliable sources for you. Say you're interested in the Bogomils, the "heretical" religious sect that arose in Bulgaria . . . and you're wondering whether they played a role in Kievan Rus. You might find a web page or a Wikipedia entry on the Bogomils. . . . and you'll have to wonder "Is it accurate?"

Much better is to consult the academic literature, for example

FINE, JOHN V. A. “Were There Bogomils in Kievan Rus'?” Russian History, vol. 7, no. 1/2, 1980, pp. 21–28. JSTOR, www.jstor.org/stable/24652309.

You'll note the "JSTOR" link. JSTOR is a database and search engine for academic publications. You can get somewhat limited access to it by registering-- for the full access you'll need to check with a librarian at a public or university (or some high school) libraries. It's worth the effort. Historical work that's supported by research in peer reviewed journals is much more compelling than what you might find on blogs tumblr, etc.

As an example of the kind of material you'll find

KOLLMANN, NANCY SHIELDS. “Collateral Succession in Kievan Rus'.” Harvard Ukrainian Studies, vol. 14, no. 3/4, 1990, pp. 377–387. JSTOR, www.jstor.org/stable/41036394.

KAISER, DANIEL H. “Reconsidering Crime and Punishment in Kievan Rus'.” Russian History, vol. 7, no. 3, 1980, pp. 283–293. JSTOR, www.jstor.org/stable/24652415.

Franklin, Simon. “THE EMPIRE OF THE ‘RHOMAIOI’ AS VIEWED FROM KIEVAN RUSSIA : ASPECTS OF ΒΥΖΑΝΤΙΝΟ-RUSSIAN CULTURAL RELATIONS.” Byzantion, vol. 53, no. 2, 1983, pp. 507–537. JSTOR, www.jstor.org/stable/44170831.

The authors of these pieces are experts. So for example, on the Bogomils, the author was John V.A. Fine, Jr; one of the most distinguished scholars of the medieval Balkans-- if I wanted to know something about the Bogomils, he'd be one of English language scholars you'd ask.

Professors Simon Franklin, Nancy Shields Kollman and Daniel H. Kaiser are similarly well regarded and oft cited in the field.

In other words, better than a blogger.