hey shroomie, this is Josh, I helped you while your tank was crashing. Ok, since your tank is still showing nitrites, you DO NOT have a full cycle completed. Patience during this stage will lead to much success in the future, as I'm sure many experienced hobbyists will tell you. The reason your tank can go through a cycle (although not so drastic) when you add live rock/sand is the amount of die off from shipping the rock etc.. As you know live rock is encrusted with beneficial bacteria/organisms that help naturally filter your water. During shipping, or simply the changes in parameters from the dealers tank to your own can cause some of those organisms to die off causing a rise in your ammonia/nitrite levels. Put all the liverock/livesand you want in the tank now and let it sit and test it every few days. Once all of your levels read zero, you can now have that blenny (or any other fish) you so badly crave lol. Until then, if you decide to throw a fish in there, you will encounter the same situation you were in previously. Hope this helped,. as always, shoot me any questions you have.
-Josh