Sudden bruises: when should they worry? The case of Michele Merlo

The death of Michele Merlo, a 28-year-old artist from Vicenza, a former competitor of Amici and X Factor, has rekindled the spotlight on what could be a sign of a leukemia such as the one that struck the boy, the bruise. The artist presented bruises on the right deltoid and left forearm, treated as muscle tears.
In which cases the bruise can be synonymous with Health problems? What are the signs to watch out for?