I don't really have blind spots but totally blind regions.... Really, as someone who never studied history beyond high school, my knowledge is mostly limited to things I have learned out of interest.
I have discovered that I am fascinated with the history of the Americas between 1100 and 1770-ish, and thus a lot of the research I have done has been focused on that region/time period. I'm definitely much better with North America than South America, and within North America am better with the land that was to become Canada than the land that was to become the US and Mexico.
Through reading other people's TL's, I have discovered that I have almost no interest at all for PODs before 1100, and PODs between 1100 and 1450 only really interest me if they involve a surviving Vinland, or other earlier transoceanic contact (e.g. The Horse and the Jaguar). My favourite TLs are ones that have a POD before 1800 but also make the attempt to show what things will be like in the 20th century (with lots of butterflies).
I am particularly turned off by military history, as I have never had much understanding of it, and also have little interest in the AH exploits of OTL historical characters. I'm more interested in the new characters, new cultures, and new nations that result from the POD, so things only really get interesting for me about 50 years after any given POD.