Is it actually possible that Gatiss put in the "blud" remark without being aware of the secondary meaning, that of an evil fairy in slavic mythology? The fairy, "blud," means "wanderer" - "an injurious fairy causing disorientation" - and leads people aimlessly round and round.
There is so much attention to detail and so many subtle references/double entendres in the Sherlock series, that I can't imagine Gatiss wasn't aware of the double meaning (or Sherlock, for that matter). In my opinion, it was a portmanteau-sort-of compliment-insult by Sherlock, meaning:
a) English slang "blud" - meaning "bro," or "mate" - which, as someone pointed out, is in keeping with Sherlock's former use of street slang, "laters," which sounds so tongue-in-cheek coming from him.
b) The alternate sense of the slang word, "blood" - like, blood brother. Sherlock and Mycroft are blood.
c) The evil slavic fairy causing disorientation and leading a person round and round aimlessly. Mycroft had said shortly before the "blud" comment that he had learnt Serbian, and that it has a Slavic root. Can the fact that there is a slavic fairy of the same name (and spelling) be coincidence? If so, it's a very lucky one. Mycroft is the master puppeteer and has a hand in everything. At the end of "A Study in Pink," Sherlock agrees with Watson's idea that Mycroft was a "criminal mastermind." And Sherlock hates being controlled by his brother (in previous episodes, he stubbornly refuses to help Mycroft). Mycroft hates "leg work," so he demands that Sherlock follow up on cases for him. As soon as he gets him shaved and cleaned up, he sends him off to uncover a terrorist network. Like an evil slavic fairy directing the actions of people around him.
In addition, I think it notable that Mycroft referred to Sherlock twice before as "brother" in the scenes leading up to the "blud." Mycroft had also told Sherlock that a "small thank you wouldn't go amiss." Sherlock refused to thank him. But right before Sherlock says "blud," that girl agent whose name I can't remember (Anthea? Not her real name) said, "welcome back, Mr. Holmes." But Sherlock said "thank you -" and turned to his brother to say, "blud." To me, it seems fairly conclusive that Sherlock was simultaneously thanking Anthea for her comment, and Mycroft for rescuing him; but at the same time as he is thanking him and referencing their blood relation, he can't resist inserting a little Slavic dig that only Mycroft would pick up on, IN ADDITION to the street-slang meaning of "bro."
So clever, I almost can't believe it. Even without the Slavic part, it is clever. Gatiss is a genius. I admire the heck out of him. And I absolutely love his posh-Eton-boy smug attitude.