Just based on the word, I would think it would mean that the historian in question is changing or revising the official account of some event, in order to make one party look better--similar to what I've read about Soviet Russia altering their history textbooks depending on who's in power. Is this the case? Or does it mean something else?
Historical revisionism is, by definition, an interpretation that challenges the standard or traditional school of thought regarding a given historical phenomenon. It is neither a positive or negative term in this context, and it does not necessarily serve some political motivation or make one party look better in comparison to another.
There are many, many examples of revisionism, and sometimes the revisionist interpretation directly influences consensus. For example, within the field of Cold War diplomatic history, the "traditional" school of thought in American scholarship was that Stalin caused the Cold War by seeking to expand the Soviet empire. The "revisionists" of the 1960s and onward argued that Stalin was simply responding to threats to the Soviet Union and that an American desire to dominate global politics and markets after the Second World War caused the Cold War. Today, a so-called "post-revisionist synthesis" has become the standard interpretation in which scholars have taken a more nuanced approach by not placing the fault on one party or the other (although one of the more prominent "post-revisionists," John Gaddis, argued after the opening of Soviet archives that the Soviet Union really was to blame -- I argue that Gaddis is really a traditionalist anyway). Here the revisionist school of thought bore great influence on the historiography of the topic.
Other times, revisionist history has no influence on consensus. For example, Vietnam War revisionists (and there are a few prominent ones) argue that the Vietnam War was either effectively served American interests or that the war was not morally wrong in the way that it seems now in the American historical memory, i.e, that the Vietnam War was not a mistake. Despite this work, the consensus did not change at all. Historians almost universally agree that the Vietnam War was a mistake, although the arguments and methodologies for coming to this conclusions vary widely (the disagreements as to why the war was a mistake only underscores the consensus that it was a mistake).
As you note, the term "revisionist" does have a negative connotation despite being neither positive nor negative in itself. The reason is that many times revisionists do have political, moral, religious, etc., motivations behind their revisionism. The "Lost Causers" want to erase the role of slavery in the Southern decision to secede because of pride or because they want to challenge federal discrimination laws today. A Vietnam revisionist might want to convince others that it is unnecessary to challenge American military decisions today. A white-supremacist might want to portray Nazi Germany as a victim rather than as the aggressor or to downplay the extent of the Holocaust in order to promote their racial agenda today. This all strikes at the heart of two of the greatest challenges that the field of history faces: our biases today affect our interpretations of past events, and historical memory is often influenced by its utility of a political tool. Because so many revisionists look to change historical interpretation to serve their agendas, the term "historical revisionism" has taken on a negative tone in political discourse exactly as you portrayed it. Because of this, a historian who challenges traditional or nationalistic interpretations of history, whether or not political considerations motivated them, can expect to be pejoratively labeled as a "revisionist" by those who embrace the traditional interpretation of events.
Hi, there have been several in-depth threads on the term that you may be interested in..
What is revisionist history? Why is it a bad thing? - featuring /u/sowser, /u/domini_canes and others
Has the term "historical revisionism," or its counterparts, always had the political implications that it currently does? - featuring /u/baronzaterdag
Open Round-Table | What we talk about when we talk about "revisionism"
and just for fun, a few examples of revisionism in action
Historians of Reddit, what topics are currently being "revised" within your field of study?
Tuesday Trivia | What's the most defensible "revisionist" claim you've heard?
Most of these posts have been archived by now, so if you have follow-up questions for any of the users, just post them here & tag their username to notify them
In the field I am most familiar with, seventeenth century British Isles history, I would say 'revisionism' is broadly used as a positive term. Specifically it refers to a wave of historiography in the 70s and 80s that argued for an increased focus on religious issues that were largely being ignored by the dominant schools of the time; the Marxist economic interpretations of people like Christopher Hill or the strongly political focus of people like Quentin Skinner. This isn't to say Hill or Skinner were wrong, they are both, Skinner especially, still highly respected and their work often well thought of, but I don't think you would find many seventeenth century historians who would deny the revisionist scholars made a very worthwhile input.