I've heard agents say that unless the comment is from someone they trust, they pay them no mind. A well-know author would probably fall into that description.
However...
It doesn't really matter that much. What someone said about your last book has little to do with how good the book you're currently querying is. If the book sounds great an agent will take a look at it, blurbs or not. If they don't like it, they won't, no matter how many great blurbs you've gotten.
Blurbs and good previous sales
can help if the agent is on the fence about the query. They can give you an edge so the benefit of the doubt falls in your favor. An agent might be willing to take a chance if others really liked your past work and it sold well, but they're just not sure about it.
And yes, it's commonly called a blurb. It is an endorsement, but that's not the term used to describe it. Endorsement feels too blatantly advertise-y I suppose