I think it's an interesting idea, and may be an interesting blog, if your writing skills are compelling and if you don't undermine the integrity of your observations by allowing personal biases to sway your powers of observation. Tall order there.
I have visited many, many churches and many denominations in various countries. Very few of them are paranoid or secretive, and even fewer would push on you if you simply came to a service, sat quietly, and observed. If asked, introduce yourself and just be honest - simply say you are visiting different churches and local places of worship to learn more about their beliefs and practices. Just be polite and discreet - no need for any sort of covert actions.
I can only speak for my church (a mid size Methodist church in a small Texas town) but I can pretty much tell you what would happen if you visited our church. You would witness a one hour service - complete with open communion if you came on the first Sunday of the month or a holiday - with a mixture of contemporary and traditional music. You would see a mostly white congregation, with a scattering of various other colors and ethnicities. You would see a lot of kids - which is a sign of health when it comes to a congregation. You would see a wide range of clothing, vehicles, hairstyles, etc. You would hear bible readings and then a positive message based on those readings, with practical real life applications. You'd spend your one hour in a 1930s built building which has been kept up very well, complete with stained glass windows and a pipe organ, in the style of a country church in England.
If you stuck around afterwards to talk with people, you'd find that the Methodists in this area are a mixture of Democrats and Republicans - what holds us together more than politics would be a shared belief in supporting good works and missions, including ministering to the children in our own congregation and community (big emphasis there) and - valuing a strong work ethic and individual freedoms. This is, after all, an oilfield town in East Texas.
Methodists are pretty independent minded. If you flat out told a group of parishioners that you were an atheist, they'd probably slap you on the back, laugh, and say, "Stick around, son - we'll work that right out of you!" and then go on to the next topic - like, "Why d'ya think the high school wasted so much tax money on that stupid indoor football field?"