If you mean comments when you say "DocBlock" or something like javadoc-style comments like:
/**
* Comment
*/
// Comment
Then this is not possible.
But you can create Annotations and use this as a doc:
@Doc("Some Documentation")
class SomeClass {
}
TypeMirror tm = reflectType(SomeClass );
print((tm.metadata.first.reflectee as Doc).string);
But I guess this is not what you are looking for?
There is a Comment class in dart:mirror - but note that it is not used and dart:mirror is officially "unstable".