Its because the 'normals' on the object you used to boolean were inside out maybe? dave? turn OFF 2 sided lighting wheneer you model to see the reversed normals. In your view panel 'Lighting>2 Sided Lighting'. If ANY object ends up with a black lambert then the normals are reversed...you need to correct this.
cheers bullet
bullet1968
"A Darkness at Sethanon", a book I aspire to model some of the charcters and scenes