There is a study demonstrating that cocoa, which differs chocolate from other sweets is not responsible.
Study in question is Fulton JE Jr, Plewig G, Kligman AM. "Effect of chocolate on acne vulgaris." The Journal of the American Medical Association. 1969 Dec 15;210(11):2071-4. Cited in PubMed; PMID 4243053.
The 1969 study was aimed at
determining whether chocolate and acne
are related. Two groups were given
chocolate bars. One group was given
chocolate bars with zero cocoa
content, and the other group was given
chocolate bars with a huge cocoa
content. The two groups fared equally
well as far as their acne was
concerned. However, the chocolate bars
both had sugar, milk, etc. The study
was only testing cocoa in particular.
However, what's nowadays called "chocolate bar" has actually very little chocolate at all. It's mainly sugar in various forms. For example Mars bar is 60% sugar (35g of 58g).
Sugars are known to induce acne:
high-glycemic diets, with their higher
carbohydrate and sugar contributions,
generally do give rise to increased
plasma levels of insulin.
High-glycemic diets also promote
increased blood levels of another
substance: free insulin-like growth
IGF-1 is thought to promote acne in a
similar way to insulin, by promoting
follicular obstruction through
inducing androgen-mediated sebum
production, but also hyperkeratosis, a
thickening of the outer layers of
skin, and epidermal hyperplasia, a
proliferation of skin cells. A study
of post-adolescent women, ages 20–25
years, found increased levels of IGF-1
in those suffering from acne.
So given that it's fair to say that eating excess of candy bars (aka "chocolate" bars) could induce acne.