Change blogspot title format for posts

1) Go to Design -> Edit HTML

2) Locate section "<title><data:blog.pageTitle/></title>" and replace it with the following:

<title>

<b:if cond='data:blog.pageType == "item"'>

<data:blog.pageName/> - <data:blog.title/>

<b:else/>

<data:blog.pageTitle/>

</b:if>

</title>

This will display the title for post pages like this:
"<data:blog.pageName/> - <data:blog.title/>"
Post 1 title - Main blog title

Choose Preview & Save Template or just Save Template.