For reference you might want to read the definition of "all the more so". You can say something has a particular quality and then follow that by adding that the quality is "all the more so" for a particular reason. Or, as in your example, you can say it in one statement by using a comparative adjective (eg one ending 'er', such as tinier, smoother, faster etc) and say "all the [smooth]er for...".
In your example, 'all the blacker' means that the aforementioned 'gulf' was more black because it has glittering walls. You would take from this that, even without the glittering walls it would be black, but more so because of them. What that means in real terms is either open to interpretation or requires the wider context of the book.
A comparable example using the same syntax would be "the cake was all the sweeter for having extra sugar in it".