private constructors and inheritance

[John Colvin via Digitalmars-d-learn]

//blah1.d

class A
{
     private this(){}
}


//blah2.d

import blah1;

class B : A {}


$ dmd blah1.d blah2.d -lib
Error: constructor blah1.A.this is not accessible from module 
blah2

Can someone explain why this can't/doesn't work? Thanks.