Blud is pseudo-Jamaican slang used by annoying teenagers who want to pretend they are in gangs. Similar to when Americans who were into rap called each other "G". The phrase originates from the Jamaican patois phrase blud clot.
Blud was very commonly used by teens in London in the 90s and 2000s - not just gangsta wannabes. Although it was annoying. Blud/blood clot/clart was less commonly used by white people in my experience but is still used by black people of Jamaican decent. You see it at protests even now:
I read ong as a typo of omg, maybe I'm old now tho. But the 3 dead skulls suggests a high level of being made dead by whatever the original comment was, which combined could be translated into 'quite shocking' among other various meanings
When you put it all together with the skull emoji (which is used to indicate you died laughing) it basically means “lol I can’t believe this dude is being serious”