The only people who dream about asking a builder for a VAT bill in Spain are people who have never been, or have tried it and scarpered back home as soon as they found the country wasn’t for them.
So far, nothing black from me. Both architects I interviewed were insistent that everything be legal.
The only time I would consider ‘black’ is if I see someone in the street, obviously starving or struggling and I have work that can be done by them. I will pay whatever the legal rate is, but obviously they have no capacity to register, pay VAT, etc.