He appears in Sharpe's Rifles, a deeply religious man, he is very conservative, a believer in honor, he is the rare creature: an honest man. He earns Sharpe's respect.
He woos Louisa Parker as well as undertaking his mission to fly the gonfalon of Saint James in defiance of France.
In Sharpe's Devil he is reported missing while serving as Captain-General of Chile. Louisa approaches Sharpe about going to Chile to find out his fate. Sharpe assumes he is going to the new world to bring back a corpse, but the facts were much stranger than his assumptions.
Don Blas had met with rebels under a flag of truce thinking they wanted to treat with him, they had mistaken him for his Francophile brother, however, and assumed he'd be interested in a plot to free Napoleon. When the discovered their error, they held him prisoner on a remote island while their plots played out.
He was shocked and dismayed that anyone would so abuse the flag of truce and stated his intention to retire from service in a world he no longer understood.
In the TV version Vivar fights alongside Teresa, and while heartened to see the budding romance between Sharpe and Teresa, also warns Sharpe not to break her heart.