Despite its enigmatic name, there's little more than a bluish haze hanging over this rubbly nature park. But these hills have revolutionary history: within their folds once hid the haidouks, the freedom fighters who plotted against the Turks during Bulgaria's centuries of Ottoman rule. Most visitors take the chairlift to the top and picnic overlooking views of the valley, or follow trails to the caves in which haidouks planned acts of sabotage against the Ottomans. The best time to visit is between May and September.
As an alternative to the chairlift, you can walk uphill following a winding trail (1½ hours). From the top of the chairlift, a path leads about 300m to the main road; cross it and proceed 500m through the woods. Follow the road downhill to bypass information centres (pick up a map). Ragged, often steep trails to the haidouk caves are sporadically signposted from the main wood into the wilds.
The park is also home to rock formations with legends as strange as their shapes. Halkata, 'the ring', is thought to weave love spells from its 8m stone bulk; stories say that passing through the circle ensures a couple long life and happiness. Even more eyebrow-raising is the claim that passing through the circle with a willingness to change sex can transform a visitor from male to female, or vice versa. Other rocks are thought to resemble camels and frogs; get more information from the tourist information centre.
Irregular minibus 25 from central Sliven reaches the main road, a short walk from the lifts, but it's far easier to get a taxi (4 lv). Keep a taxi number for the way back, as they don't reliably ply the lifts.