> I don't claim to be an expert at this but I thought the mass of the sun was too low to form a black hole.
"around the size of the sun" generally refers to 'stellar-mass' black holes. Black holes are generally divided up into 3 categories (4 if you include the micro blackholes that get created and evaporate within seconds):
1) Stellar-mass black holes, anything less than about 100 solar masses;
2) Intermediate black holes, 100-100,000 solar mass;
3) Super-massive, generally 100,000+ times the mass of the Sun.
These categories are fairly loose, they are approximations.
Therefore often whey they say 'about the mass of the sun', they generally mean stellar-size black holes, i.e. anything less than 100 solar masses. When you have supermassive black holes at 10 billion solar masses or more, then relative to that, a 50 solar mass black hole is "about the mass of the sun" when engaging in general discussion.
Also note that the initial mass of a star has to be much greater than the Sun's to form a black hole, but that doesn't mean the resulting black hole will be the mass of the parent star. A Supernova is caused by a large star ejecting much of its mass, so as part of the supernova/collapsing into a black hole, most of the stars mass is blown away into space and doesn't become part of the black hole. (at least in the star->supernova->blackhole mechanism, there are several black hole creation mechanisms, such as direct collapse and others)