When we want to refer to something that is totally backward, asinine, or outdated, we use the phrase "something out of the dark ages." As descriptive as this phrase might be it is, in itself, "something out of the dark ages." That is, it's mostly wrong. The dark ages were a time of political, and military upheavals, but then what age isn't (our own included)? The point is though, they were not dark, at least in an artistic, literary, or scholarly sense. The worst that could be said of the period from the third century, when the Emperor Diocletian split the Roman Empire into two parts (eastern and western) in order to save the eastern half from the chaos and destruction that befell the western half, to the beginning of the early Renaissance in 1400, was that it was primitive (but hardly any more so than it had been in the eastern Roman empire before this time). And while there wasn't exactly a "flourishing" of the arts, they continued to exist and develop even amidst the political and military upheavals I mentioned before.
We call the artistic period during this time Byzantine, for its cultural and political centre, Byzantium (now Istanbul). Here for instance, the ancient Roman art of mosaic images continued to develop as seen in the San Vitale, Ravenna, murals of Justinian and His Attendants or its companion piece depicting his queen, Theodora. Although there were problems involving those who thought any depictions of Christ, God, or Mary were idolatrous, and a period when these Iconoclasts (image breakers) wreaked havoc with figurative expressions in art, there can, nonetheless, be seen a fairly constant development of artistic styles and skills during this period.
And there continued to be painting, as well. Even in the West there was some fresco painting being done (admittedly crude) in churches as a means of educating the illiterate believers at the time. A David and Goliath fresco at Tahull, Spain, done about 1123 is an interesting example. It has a linear, cartoon-like quality to it, and a limited, rather flat colour range, but it's still quite effective for its decorative/educational purposes. And in the East, painting took a different, more personal direction in the form of icon painting--small, religious, panels depicting, usually the Madonna and child--which, as they developed, became both beautiful and quite sensitive in the emotional interaction between the mother and child. Later, with the spread of Christianity northward to Russia, these deeply religious evocations of beauty and faith developed still further, and are perhaps the best evidence that the so-called "dark ages" weren't as dark as we often think.