I know very little about Hoya sp. Black Leaves EPC-301 other than it has an Epiphytica accession number and hails from Prachinburi Province in Thailand. As you can see below, the leaves are far from black. The only thing I can suppose is that new growth when grown in extremely bright light, may come in a very dark color. I for one have not seen it. I have however seen a strong reddish tint to the leaves and good coloration with dark leaf margins on new foliage.
The flowers are typical of at least a dozen different Hoyas that I can think of and are highly perfumed. I received Hoya sp. Black Leaves EPC-301 in a trade from a collector in North Carolina in the summer of 2015 and flowered it in the summer of 2017. The plant grew in my regular mix and I would put the difficulty level with 1 being dead easy, and 10 being impossible at a 4.
Although the flowers are similar to many other Hoyas, Hoya sp. Black Leaves EPC-301 has its own charms with beautiful, shiny, speckled, leaves and an outstanding scent. If you don’t own another plant with the same type of flowers, I can recommend picking one of these up. It flowers in two years from cutting and will produce a very nice specimen plant in a short time.