Substitution in vim is, by default, case-sensitive. Adding the is-flag makes it case-insensitive. vim-abolish, on the other hand, lets you perform a case-insensitive substitution that preserves three case variants (foo, Foo, and FOO). Substitution with vim-abolish can be performed with Subvert or S.
For instance, :%S/blog/article/g will turn
blog Blog bLOg BLOG
into
article Article bLOg ARTICLE
Install vim-abolish and see :h Subvert for more details.