I'd been wondering about the absence of blossom trees in my game, so I started a creative mode game with 20x speed for testing. In the middle of a forest, I spawned some tiberium nodes with seed missiles, then put growth accelerators on them. The entire field grew by 200-300k per cycle. All that happened to the trees was that they got destroyed during growth of new tiberium. No blossom trees were visible anywhere, and the only nodes being created were the usual ones somewhat away from the edge, in high concentrations.

What's the condition for blossom trees being spawned?

Blossom trees should happen on each node, not just from trees being infected.

Also, nodes from trees still follow the same rules of growing a new node. There's a minimum distance to other nodes, so as to not have tons of nodes that kill UPS. The tree, IIRC, gets absorbed into the tiberium, giving a growth boot on the tile that absorbed it.

So, if I understand correctly, an infected tree will not spawn a node or otherwise visual indicator, but just give a direct (one-time?) boost to the tile where it stood before?

There is a chance it can spawn a node, if it is far enough from other nodes. Otherwise, yes.