At the moment, ascended Black Cat (& ascended Nightcrawler I presume) are not giving out retroactive rewards for Silver Sable. Per their own policy, there should be a 4 days period where those ascended characters would give out rewards depending on the level of said character.
This was fixed last night, ascended black cat gives sable covers and shards, nightcrawler givess 2ndary sable shards AND archangel covers and shards - which is a nice bonus.