Please tells us what bios settings you are trying to run and the hardware set up. Also as a general rule it is advisable to always monitor temps when stress testing a cpu. Real Temps or some other temp monitor should be run during the test. Is hyper thread enabled, turbo boost, speed step, etc.? All threads should pass any less indicates instability of the settings. What memory and settings are you using. Blend uses lots of ram. Use all bios set at default as a start point to be sure you can cool and run the stress at basic settings.
Ok so here is your answer. Your system is not stable by all usual definitions. Yes when we say our OC is stable it usually means it has run all cores on all threads and passed whatever test we are using. As an example, my 920 is OC at 4ghz and ran prime blend for 24 hrs with no errors on any thread. All my temps were at or under 72c. Now your system may run just fine in your real world use where it never gets pushed as it does in prime. Please understand that it is usually heat that causes instability. The causes of the heat can be various.