It is several years since we last saw a song thrush in our garden. Although a large garden, blackbirds terrorised any song thrushes they encountered and drove them away. Barbara Johnson.
Both species are thrushes (genus Turdus) and all thrushes are very aggressive when defending their territories, but blackbirds hold their territory during the winter while song thrushes do not. Perhaps it is during the winter months that your blackbirds are chasing the song thrushes away.
We
This was some years ago so I’m afraid I don’t remember if it was during the winter. We only ever saw one thrush over a period of time but after the blackbirds drove it away there were never any more! Barbara Johnson.
Southwick Country Park Nature Reserve 
It is several years since we last saw a song thrush in our garden. Although a large garden, blackbirds terrorised any song thrushes they encountered and drove them away. Barbara Johnson.
Both species are thrushes (genus Turdus) and all thrushes are very aggressive when defending their territories, but blackbirds hold their territory during the winter while song thrushes do not. Perhaps it is during the winter months that your blackbirds are chasing the song thrushes away.
We
This was some years ago so I’m afraid I don’t remember if it was during the winter. We only ever saw one thrush over a period of time but after the blackbirds drove it away there were never any more! Barbara Johnson.