Sex
It is chav speak and means friend/mate, it is often used after another chav word, such as 'innit' - innit blad. Nobody really knows what it means, but if you want to be down with the kids, you should use it lots. Kids will praise you for it
Blad is a bit similar to bro however bro is short brother as well as a chavvy name for mate, usually a male friend, use-"alright bro" and blad is normally used as a male friend but not as a brother use-"init blad"