noch can not only be used with comparatives, but can have a different meaning, i.e.: "Von den bisherigen Tagen war das noch der schönste" = The day wasn't really nice, but compared to the rest, ist was the nicest.
Another example: "Von den T-Shirts sind das noch die günstigsten." = The t-shirts aren't really cheap, but they are the cheapest
Hi, I don't know Heidegger, but I think there are two possibilities: 1. You can translate "noch" with "until now" here. It means, until now, the 19th century was the darkest century, and he implies that there could come darker ones in the future. OR: 2. Maybe he thinks that none of the centuries have been very dark. But if one has to be called dark, it would have been the 19th century.
Which of this possibilities is the right one depends on what else he says about it.