Hi Mirian, You should say "I realized my mistake when I had a look at the blog". You need to use the Past Simple of the verb 'realize' because 'when I had a look at the blog' is a finished time period (just like 'yesterday', '2 hours ago', 'in 2005' etc). 'I had a look' and 'I took a look' are both possible. Jamie (www.italki.com/jamie.teacher)

Both are correct, but since "when I (did something)" refers to a very specific point in time, you should use Past Simple: I realized my mistake when I took/had a look at the blog. I've (I have) is Present Perfect. We use it specifically to say that an action happened at an UNSPECIFIED time before now.