The actions of an unlawful assembly and the punishment thereafter are set out in the subsequent provisions, after Section 141 of the IPC, and as long as those ingredients are met, Section 149 of the IPC can be invoked.
Indian Penal Code
The Court will have to come to the conclusion based on the facts and circumstances of each case, and there cannot be any fixed formula that the prosecution has to prove, or need not prove, that the blood groups match.