Glad you got your thing working. The
default is a quick one-liner, although some argue the
is/then/else is better on performance, especially when the double brackets are exploded
So, you don't have to use the default modifier. Try this call:
<img src="[[+tv.blogThumbnail:is=``:then=`assets/templates/default/img/logo_blog.png`:else=`[[+tv.blogThumbnail]]`]]" />
Or to be more efficient? Chuck the default image url into a tiny chunk [[$defualt-image]]
<img src="[[[[+tv.blogThumbnail:is=``:then=`$default-image`:else=`+tv.blogThumbnail`]]]]" />