It was very difficult!
All things were easier to do when you went along with the tide, but if you went against it, it became difficult! It was the same for the blood veins. They would appear one by one as a person circulated the Qi in his body. It was also possible for all of them to appear at once in an explosive force. That was easy to do.
If someone wanted to call back the blood veins one by one, then they would need to control the speed of the Qi in their bodies with extreme precision. Su Ming would have to control his Qi at a fine state to arrive at that level of precision.
It was extremely rare for a Berserker at the Blood Solidification Realm to do so. This was in fact, not something for a person still in the Blood Solidification to do. This was a skill for a strong Berserker in the Awakening realm to understand and perform.