On the Swiss Hiking Scale, the ascent difficulty would be graded T3.
0. General: From 760 m to 1844 m, exposition N, mainly marked, but on Lisec ridge only a weak, not marked trail and scramble. Gear: good shoes, poles.
1. Effort: 1200 m, 3:30 h.
2. Power: 1 - no difficulties.
3. Psyche: 2 - easy, steep grassy slopes require some care, also the easy scramble below the main ridge.
4. Orientation: 3 - medium, you search for best passages in the area of Lisec ridge.
Here, I'd like to describe shortly an interesting route on Črna Prst, which is nice to do as a round tour, descending then by the shortest route via Za Liscem pasture or Za Črno goro pasture.
The optimal trailhead is Ravne above Bohinjska Bistrica, where we can leave our car on 760 m of altitude. The other option is Polje village (or Žlan, a bit SE of it), where we park on some 540 m.
The mountain road, which crosses the whole northern slope of Črna Prst (and further towards the West) goes from the ski resort towards the West, but we soon catch the marked path and deter from the road to the left, up the slope.
Through a forrest we go allways up, the direction is SW, until we reach the upper road, coming from the left. We are on the altitude of some 1040 m.
By the upper road we continue towards the West, reaching a broad valley, where the normal ascent via Za Liscem pasture deters towards the South. But we continue around the valley by the road, and around the ridge of Lisec summit into the next valley, coming down from Osredki pasture. Here, on the altitude of some 1100 m the marked path from Polje comes from the right and continues towards the South.
We leave the road and by the marked path reach Osredki alpine meadow, 1400 m. Here we can choose:
If we choose Lisec summit, we must find a steep, not marked path, which goes from Osredki meadow towards the East, over the slopes of Lisec. It is a bit hard to follow, steep, but besides some obvious care we find no real difficulties. Through the best passages we reach the side ridge of Lisec, 1653 m, the panoramic summit is just a few minutes towards the north.
From Lisec summit we continue towards the South, following the ridge of Lisec. Again no difficulties, but also the path is more and more lost. When we approach the main ridge of Črna Prst, we already see the marked path, coming from the right, go on the path and ascend the main ridge in the area of Vrata, 1721 m.
Then towards the East by the main ridge path for another good 100 m to the hut and the summit of Črna Prst.
You best descend by one of the shortest routes, either via Za Liscem pasture or Za Črno goro pasture.