It's been a while, but last I knew, blocking was not a 100% be-all-end-all never-see-you-again solution. While it does tremendously reduce the likelihood of encountering a blocked player, there instances and situations where you can still wind up instanced with a blocked player such as:
When that blocked player is part of a wing, and other players in that wing are not blocked, or one or more of the players in that wing are Friends listed.
When the blocked player is the only other player available.
I recall there being an entire paragraph on the subject, and it was as convoluted as most things Elite, but in the end, it boiled down to basically saying "Blocking someone mostly works, but isn't 100%."
This may have changed since then, I dunno. To date only two people have ever made it to my blocked list, as well as having their IP range added to my router's blacklist - sorry for anyone else in that range if they're dynamic, we won't be crossing paths. I also blacklist IP ranges from certain countries too - sorry Lil' Fattie, your nation is one of them, along with most of the middle east, all of Somalia and Myanmar.