I'd not heard of the second type you mention - got any links?
Black SLINK is generally , as you say, about 8-9mm dia, polyurethane coated, and made up of a shielded, litz-like construction. A stripped -down version without the shielding, drain-wires, Gore-tex and with fewer cores, was sold as 'White SLINK'
There was also an early , solid-core iteration IIRC.
The 'Black 16 ' and '500' type speaker cables are much, much stiffer than Slink though, being multiple wires extruded
inside each core of solid ptfe insulation. Perhaps it's been confused with black slink somewhere?
One day I shall get around to taking pics of all the bits I seem to have accumulated and
postingthem...
[ If all this doesn't confuse you enough - it's also possible to have Black Slink as speaker cables, but
very rare due to the extreme cost - one needed two runs per speaker if you ran '50S' amps..!]