Is it valid to form the following triple:
_:bn rdf:type foaf:name
where _:bn is a blank node?
I read the W3C standards for rdf:type. It says that the rdfs:domain of rdf:type is rdfs:resource. rdfs:resource is the the class of everything.
So is it correct to assign a rdf:type for a blank node?
Yes, it's absolutely fine. Blank nodes are simply things without a URL identifier. (Well it's a little more complex, but I wouldn't worry about it)
Like a car without a registration plate, it doesn't stop them doing anything cars with plates can do.
But it makes life difficult for people trying to work out whether they've seen the same car, or find the car.