...but the downside is that the default-value is not declared with the attribute but buried in the BUILDer-code and to make this work the attribute has to be "rw".

How about using the initializer option?

use strict;
use warnings;
use 5.012;

package Dog;
use Moose;

has color => (
    is            => "ro", 
    initializer   => sub { my ($self, $attr_val, $setter) = @_; 
                           defined $attr_val or $setter->('black');
                         }
);

my $dog = Dog->new(color => undef);
say $dog->color;

--output:--
black
[download]

I am not so much interested in the feature itself, but it is interesting to see some examples of Moose-metaprogramming at work...