An excellent little handbook on medieval historiography, and the source that I am primarily using in writing this response, is What is Medieval History? by John H. Arnold.

The concept of the "dark" middle ages emerged as early as the fourteenth century with Petrarch, and was the result of a historiographical tradition of dividing history into three parts: ancient, medieval, and modern. From the beginning, the idea of a "middle age" had a negative connotation. It was believed that the dark middle age was the long age of ignorance and darkness, and it was only with the development of the renaissance, and the flowering of the arts and humanism that came with it, that constituted the restoration of the great classical past. This stereotype of a "dark age" full of grime, ugly art, illiteracy, disease, and general gloom has persisted until today.

The historiographical "turn," as it were, can first be pinpointed to the German tradition, particularly Leopold von Ranke (1795-1886) who in many ways defined the professionalization of history as a field involving analysis of primary source documents and narrative history. von Ranke was not specifically interested in the middle ages, but his methodology of primary source research influenced the extent to which preconceptions about the "darkness" of the middle ages were being interrogated, investigated, and challenged through analysis of primary texts, rather than sweeping statements without true historical evidence.

Next in terms of major historians comes Marc Bloch and the Annales School, a school of primarily French historians interested in more than political narrative histories: they wanted to talk about the longue durée, or long-term history, including the impact of geography on culture, society, economics, and even psychology. The Annales tradition was later blended with Marxism in the more intensive study of feudalisms and economic structures by the likes of Georges Duby. More recently, in the 1970's and 1980's, more specific areas of study have developed, including gender, sexuality, and intercultural and inter-religious relations between Muslims, Jews, and Christians during the medieval period.

Overall, the sense that one gets from Arnold's historiographical overview is that there has been a fairly steady progression that has involved narrowing, both in terms of methodology and in terms of subject matter. With the introduction of primary source history with von Ranke came a great interrogation of medieval stereotypes, but the focus was still very much on macrohistorical, "history from the top down" history that focused on the large-scale political machinations. Over time, there has been a greater interest in history from the bottom up, a focus on every day life, economic, racial, and sexual minorities, and perhaps a more microhistorical focus, both in terms of geography and chronology.

There is always "resistance" or dispute among historians, and medievalists are certainly no exception. Medieval historians have a reputation for being traditionalists, always a few years behind in terms of historiographical trends. There are camps that favor a very "scientific" type of history that involves graphs and statistics, while others like Natalie Zemon Davis provide a much more subjective (but arguably more compelling) microhistory that relies on fewer sources and much psychological conjecture.

Another interesting thing to consider, and something is constantly being challenged in the scholarly community, is not only the idea of a "dark" age but even the idea of a "medieval" period. Other eras of history, such as Victorian history, early American colonial history, and even many historians of classical history, tend to focus on as little as 50 years and perhaps as many as a century or two of history. But the "middle ages" constitutes a full millennium! Could anyone possibly say that the years 1000 to 2000 constitute enough homogeneity to warrant academic study as a discrete whole? Of course not. So why do we do the same to the middle ages? In many ways it perpetuates the three part division into ancient, medieval and modern, and breeds stereotypes about the middle ages as several centuries of stagnation. In that sense, then, does medieval history, and medieval historians, even exist as a valid category?

There has been a turn in recent years towards questioning and pushing the boundaries of these parameters, especially in terms of periodization of the middle ages and the so-called renaissance. Some historians have argued for an earlier twelfth century renaissance, and some believe that the existence of a "great renaissance" as a historical period belittles and denigrates the technology, art, and complexities of the medieval period, from which there are very few differences.

There have also been questions about the terminology historians use when they try to characterize the middle ages. A large debate has taken place over the use of the term "feudalism." In no medieval document is the term ever used, but if you were to take a basic Western Civ course in high school or an intro college class, it would be a large portion of what you learned about the medieval period. In many ways, terms like "feudalism" only serve to generalize. Areas as close together as medieval England and France had distinctly different economic and social structures, not to mention other parts of Europe, Asia, and North Africa. But "feudalism" has become such a common term that erases these nuances and distinctions and encourages the impression that the medieval period was one of socioeconomic uniformity.

This has been a rather lengthy and winding answer to your question, but I have a final caveat. While so far I have demonstrated that there have been historigraphical trends in terms of what interest historians about the middle ages and how they approach medieval history, there is also a distinctive way in which medievalism is represented in the popular imagination. Television shows like Game of Thrones and "reenactment" activities like Medieval Times restaurants and Renaissance faires all contribute to popular conceptions of the "darkness" of the middle ages. While historians, on the whole, no longer view the medieval period as a time of ignorance and griminess, the average person no doubt would. Think of the connotation that the word "medieval" has today. In describing 21st century combat or torture tactics or discriminatory policies against racial, gender, or sexual minorities, the word "medieval" is often used.

Don't get me wrong, for many, many people during the middle ages, life was grim, life expectancy was shorter, and what we would call "quality of life" was very different. But recognizing the alien in the medieval does not have to be a condescending exercise in creating the image of a "dark" age. Historians now study medieval subjects that vary from folk medicine to courtly culture to combat styles and everything in between. If asked if they would want to live in the middle ages, most medievalists would respond with a resounding "no," but that does not mean that the renaissance was any better than the middle ages, that the middle ages are undeserving of study, or that echoes from the medieval period don't surround us every day.