Just on the subject of eras:

I'm interpreting your question to mean what era's in history have modern historians been unable to produce substantial scholarship. The obvious answer is that the further back you go the fewer surviving settlements, written accounts, and other physical evidence remains to be explored. So using that logic there will always be a significant drop off of credible information to be gained from extremely early periods in history. That aside, the relative level of sociocultural and technological advancement in various societies across the world are vastly different during the same periods of time.

The question is difficult to answer because, for example, when the America's were being colonized the natives in the region were at a completely different stage of development than the Europeans that arrived. It's extraordinarily difficult to tell how long the native Americans, for instance, existed in the tribal state prior to the Europeans dominating the region. Considering their ability to live off the land and their unwillingness to change, the native Americans could very well have been living in essentially the same way for thousands of years.

If you mean European history, we have substantial historical records dating back to the Greek city states, but all we know of before that is based on discovered settlements. We have a relatively comprehensive history of Rome (Republic into Empire), but very little real information about the thousands of Germanic tribes and Gallic peoples that lived in Europe at the same time. Most of what we do know about the tribal Europeans comes from Roman accounts and is therefore subject to criticism. In addition, most of the origins of these societies are impossible to directly pin down as the Romans and Greeks claimed to originate from divine means.

After the fall of the Roman Empire there was a few centuries long period sometimes called the dark ages. This era was generally a period of decay for social and technological advancement in Europe and might fit your definition of a "blind spot." Inevitably, however, after Rome you get history branching in too many simultaneously occurring directions in Europe due to the many nations that pop up across the centuries. Add the Catholic Church rising to power and you get a very complex picture.

Without knowing which history you mean, it's very difficult to create a comprehensive answer.

Incorporating Environmental History approaches into general history is still an issue from my persepective, but it's also my field. In the same way that ecology can be seen as a disruptive scientific discipline, I think environmentally focused histories are capable of disrupting previously well understood and agreed upon conceptions of time and place.

Most understandings of history place the human role as the fundamental actor, the mover and shaker to the things around them. Until fairly recently the same sort of issue was present with men and women being either actors or acted upon in many histories. If instead you view the human role as being a participant in a greater system you're suddenly presented with lots of new and exciting questions. This is even more true when you consider how recursive some of the potential interactions can be.

The history of fertilizer and population density is an easy example of this I think. After the agrarian revolution productivity improvements were fairly marginal - small, consistent, and subject to the winds of catastrophe. Fertilization was broadly understood and crop cycling and reintroducing nitrogen rich manure were fairly common practice. Eventually wealthy maritime powers were able to mine fertilizers from across the globe. Then in 1909 Fritz Haber figured out how to fix nitrogen through ammonia synthesis and Karl Bosch figured out how to scale that to industrial levels. Suddenly we have modern agriculture where at least macronutrient composition of soils can be readily addressed. Productivity increases, carrying capacity increases, and we're off to the races with subsequent population increase, suburbanization, rural flight, and all other variety of change. That's not even to get into the runoff (literal and figurative) effects of that fertilization activity.

Adding the environmental component to any history makes it richer. Adding that component well and adapting your assumptions to accommodate it makes writing that history much more difficult.

Lack of translation of secondary sources to English, and small numbers of Anglo historians speaking some languages effectively creates this, as far as some regions' histories in English are concerned. I am sure that other posters will have examples from regions they take an interest in.

It must be an even bigger issue for some regions elsewhere in the world, but the availability of pre-20th histories of countries of Central Eastern Europe that are not Russia is fairly limited. A few scholars have helped to open things up (e.g. Norman Davies for Poland, Stephen Rowell for medieval and early modern Lithuania, Florin Curta for Romania and the Balkans in the early middle ages). But especially for the smaller countries, if you only read English to an academic standard, you are limited to a handful of general histories plus a few monographs on specific periods and events. You can't get the range you would on France, Spain, Germany and Italy in the same period. (I strongly suspect that it wouldn't be possible to be an AskHistorians flair for the history of some of these CEE countries if only fluent in English, as it limits access to material so much.) I don't think it's brilliant either for the Nordic countries outside the Viking era, but it's not as bad as if you were trying to do history of Bulgaria, Romania etc. Hopefully when the kids of 00s-2010s East European arrivals in Britain get to postgrad, there will be more interchange.

You might be interested in the book "Vanished Kingdoms: The History of Half-Forgotten Europe" by the historian Norman Davies. It's about the history of historical kingdoms and states that aren't discussed very often at all. Think places like Burgundy and Lithuania instead of the usual France, England and Prussia.