Completed Super Saiyan Blue is the fully mastered version of the Super Saiyan Blue form, in the same vein as Super Saiyan Full Power is the mastered version of Super Saiyan. This state only exists in the Dragon Ball Super manga.
In this mastered state, the user remains the same as the unmastered version, except that the aura is not present, as it is sealed within the body.
Usage and power
Once a Saiyan has managed to overcome the stamina flaw of Super Saiyan Blue, they can learn to seal the overflowing power of Super Saiyan Blue within themselves and thus achieve this perfected state.
A Completed Super Saiyan Blue is capable of using the full power of a Super Saiyan Blue (which normally only manifests in the initial phases of battle) at all times, and thus they are far stronger than an unmastered Super Saiyan Blue. In this mastered state, Goku was so powerful that he could match Fused Zamasu in battle and seemingly holding a slight edge in power, though Fused Zamasu's immortality gave him the edge in overall combat.
This state is similar to the Super Saiyan Full Power state in that it is a mastery of the ordinary form, however, like the Full-Powered Super Saiyan 4 state, Completed Super Saiyan Blue is different in that it does not replace the Super Saiyan Blue form, and must be powered up into when the user has enough time to focus and seal their power inside themselves.
This technique is not perfect however as it requires intense focus in order to keep the ki from leaking out of the user's body. To which, any leakage of ki will cause noticeable pain on the user from the point of release. Vegeta also indicated that such an action will cause the user's body to become unstable and shatter at any given moment.
In this state, Goku is capable of using a weakened version of the Destruction technique, by concentrating all of its power on his hand. However, for the technique to completely eliminate the target, Goku must continue his concentration on the target unlike Beerus' version which works independently, which allowed Fused Zamasu to survive the assault.
In the manga, sometime between the "Future" Trunks Saga and Universe Survival Saga, Vegeta manages to attain the Completed Super Saiyan Blue. In this state Vegeta arrogantly believed that he was stronger than Beerus, However, when Beerus got serious Vegeta was taken down in one shot. Afterwards, Beerus notes that Vegeta is now strong enough to become a God of Destruction in a different universe.