How to select elements using HTML agility pack include selector element?

You can keep using //div[@class='bla'] selector and get HTML markup of the corresponding divs from OuterHtml property, for example :

var html = @"<div>
    <div class='bla'>
      <p>Some text1</p>
    </div>
    <div class='bla'>
      <p>Some text2</p>
    </div>
</div>";
var doc = new HtmlDocument();
doc.LoadHtml(html);

var nodes = doc.DocumentNode.SelectNodes("//div[@class='bla']");
foreach(HtmlNode node in nodes)
{
    Console.WriteLine(node.OuterHtml);
    Console.WriteLine();
}

Dotnetfiddle Demo

output :

<div class='bla'>
      <p>Some text1</p>
    </div>

<div class='bla'>
      <p>Some text2</p>
    </div>