Copyright law is everywhere statutorily based. That means that you have to examine the particular copyright law in country where you live. There are 165 countries in the world, including the United States and most of the developed nations of the world that are signatories to the The Berne Convention for the Protection of Literary and Artistic Works, the basic copyright protection document. Contrary to common belief, even the People's Republic of China is a signatory. Most countries have some local version of the copyright act. For example, in the U.S. we have a series of laws (statutes) that comprise our copyright law. For an explanation of the statutes and their application in the U.S. you can get a basic introduction from the U.S. Copyright Office.
If you live in the United Kingdom, your copyright protection is governed by the Copyright, Designs and Patents Act 1988. For a fact sheet on U.K. copyright law see Fact sheet P-01: UK Copyright Law.
If you are in Canada, the copyright law is administered by the Canadian Intellectual Property Office and the Copyright Act (R.S.C., 1985, c. C-42).
If you live in Australia the Copyright Regulations 1969, the Copyright Tribunal (Procedure) Regulations 1969 and the Copyright (International Protection) Regulations 1969 specify matters related to the operation of the Copyright Act of 1968. The copyright law in Australia is administered by the Australian governments' Attorney-General's Department.
In New Zealand rights for copyright protection are provided for in the Copyright Act 1994 and Copyright (New Technologies) Amendment Act 2008. The Copyright Act 1994 is accompanied by the Copyright Regulations 1995.
Other countries have similar governmental agencies and statutes.
In addition, in common law countries, such as the U.S. and the U.K., decisions by the courts may also affect how copyright law is applied.
Now, to the question of whether or not a blog post is covered by copyright. I quote from the Copyright Office Frequently Asked Questions, "Your work is under copyright protection the moment
it is created and fixed in a tangible form that it is perceptible
either directly or with the aid of a machine or device." There is no longer a requirement that any notice be given to a claim for copyright protection. So, you can assume that anything you read on the Internet, unless otherwise noted, is subject to copyright protection.
That issue is relatively simply disposed of, but there are, of course, a huge number of issues beyond that simple statement. That is basically why there are intellectual property lawyers in the world.