A single Blackwall hitch around-a-hook may slip, but the same hitch when tied around-a-bight is much more secure - and if doubled, it is a very secure, safe, easy to tie non-jamming hitch. Se the attached pictures, for three variations of this hitch.
Those hitches should perhaps be compared with the backhanded hitch, with has about the same complexity. Another interesting and quite secure hitch-around-a-bight, is the humble overhand !
In fact, I think that this is the one and only place where a single overhand knot is so secure as a hitch - because the standing end squeezes - very effectively- the tail inside the curved U section of the tip of the bight.
(See the next post, for pictures of the backhanded hitch, and the surprizingly secure overhand-around-a-bight hitch.