"Bloom" is something mostly heard in live music, and rarely in reproduced (recorded) music (and the systems most of us have).
It is when the harmonics (overtones) of an instrument (or an orchestra) spread outwards in space, the way it does Live in a symphony hall (but not in a rock concert, which is about brute force). In most systems, "bloom" is the least likely trait to be achieved, as that is typically seen in very expensive systems. My WATTs - none of the generations I had - were not the type of speaker to produce "bloom." The WATT, in its first four generations, had much more of the "direct" sound, which is what Dave Wilson was aiming for when he recorded. Most of his recordings were violin/piano very close to the microphone, so there is not much bloom there. My Avalons and Infinity speakers did do that. It is not related to attack or decay. Think of it as a kind of "echo" in that it goes on for seconds after a string is plucked, or percussion hit (assuming they don't silence the instrument manually). But the main point is that pop/rock music rarely ever have bloom. That is the purview of classical/jazz/international music, and even THEY must be VERY well recorded.