The following is my abridged version of their emails and any errors are mine alone.

Basically what you have here is a pyroclastic breccia consisting of angular blocks of sedimentary rock and maybe even some ragged to blobby bombs of pale basalt.

The bluey grey tuffaceous matrix is probably mostly finely comminuted sediment, the local country rock is largely limestones and mudstones but maybe this has some bluish chlorite, some carbonaceous material perhaps or just a blue cast to the photograph.

Thanks to ...

Professor Barry Dawson Professor Brian Upton Dr David Stephenson