I have never played the Half-Life 1 alpha, but this is the best quick implementation I managed to make:
Add this to a monsters TraceAttack
function
if(m_bloodColor == BLOOD_COLOR_RED)
UTIL_BloodStream(ptr->vecEndPos, gpGlobals->v_forward * -35 + gpGlobals->v_up * 2.1, (unsigned short)73, 100);
(If the TraceAttack function doesnt exist then create one or you can just add this to combat.cpp
if you want this to apply to all monsters)