The sun can be turned into a black hole but it can't do it in the traditional manner.

It would require some exterior agency to crush the sun to a sphere with a four mile diameter to become a black hole with the same mass.

Normally stars much more massive than ours collapse in on themselves reaching the requisite density.

Red matter seems to compress existing matter rather than adding more mass and self compressing.

Rewinding to the fate of Romulus, radiation pressure would be a risk even to rocky structures like planets but it would have to be quite close (100 light years or so) even at 300 light years the atmosphere of the planet could be at risk but they would 300 years to figure out a solution.