You should write second for loop in you twig tamplate, like this:
{% for blog in pagination %}
{% for tag in blog.tagsArray %}
<a href="{{ path('AcmeDemoBundle_tag', { 'tag': tag }) }}">{{ tag }}</a>
{% endfor %}
<br/>
{% endfor %}
This will create a list of tag for each blog (each in separate line)
[EDIT]:
Add something like this to your Blog entity:
public function getTagsArray()
{
return explode(',' ,$this->tags);
}
This will convert tags string to array (delimited by comma). Then you can get tags array by calling blog.tagsArray
in twig. I have updated code above.