Google's index is one nebulous behemoth these days. Since they started concealing the actual size of the amount of pages they index to return results for users, it's been kind of a guessing game of sorts to figure out roughly how many different types of conversations are going on at once in the "blogosphere" that Google actually "listens" to, so to speak.
Luckily, their are tricks of the trade that Google themself gives us to take a slap-dash inventory of what the search engine considers worthy of indexation.
Let's say you work in the customer service space, let's also say that you want to know how many people are talking about topics that would be relevant to your interests, and possibly identify potential content promoters or perhaps someone who will give you a guest post. Simply utilize an iteration of the "allintitle" operator
allintitle:"Customer Service Blog"
And Google will return only results of blogs whose title tag identifies them specifically as a "customer service blog". This operator is extremely handy when gauging conversations around your industry or taking inventory of a particular topic of interest that you want to make a piece of content about (infographic, instructo-graphic, web tool etc.)
Below is a collection of some of the more voluminous categories that could be found within the index using this operator. Sourced from Wpromote.com's Google Operators For Content Marketing Cheat Sheet.