adding two hexa number problem

.model small
.stack 100h
.data
msg db 'enter 1st number:','$'
msg1 db 'enter 2nd number:','$'
msg2 db 'sum is:','$'
.code
main proc
mov ax,@data
mov ds,ax
mov ah,09h
lea dx,msg
int 21h
mov ah,01h
int 21h
mov bl,al
mov dx,0ah
mov dx,0dh
mov ah,09h
lea dx,msg1
int 21h
mov ah,01h
int 21h

mov cl,al

add bl,cl
sub bl,30h                    ; why subtraction gives the right answer?
mov ah,09h
lea dx,msg2
int 21h
mov ah,02h
mov dl,bl
int 21h
mov ah,4ch
int 21h
main endp
end main