Gravity is a force that knows no preferred scale.
Its strength is defined by the famous 'inverse square law' which has no known limits of action. The trick is whether Mother Nature can find a way to create them from the usual mechanisms at her disposal. Stellar-mass black holes seem rather easy to produce. All you need is a star more than 10 times the mass of our sun. By the time the poor thing gets to its last gasp in producing energy via thermonuclear fusion, it goes supernova and a portion of its core implodes to form a black hole with a mass of several times the sun.