that comes from drinking hard booze and smoking, working the blues bars 6 to 7 nights a week. modern vocal teachers can show you tricks. there are you tube videos out there, but getting in the trenches is (IMO) the only way to really do it. Even then only a few are successful. ive been trying for years.
I had a good voice for soul and blues with that raspiness , but when I quit drinking and smoking , my voice smoothed out . then add years of not singing ,my voice sounds like that character on family
guy (herb the perv) when he sings. not very soulful, but funny.