Because it was dark chocolate, and a bite seems to weigh 7 grams, I think you should contact a vet.
How toxic chocolate is depends on the amount of theobromine in the chocolate (the darker the chocolate the more theobromine) and the amount the bunny ate. This should be divided by the bunny’s weight to get the ingested amount per kilo or pound of rabbit. Rough estimate of bun’s weight will do:
From The Merck Veterinary Manual, 8th ed
Unsweetened (Baker’s) chocolate = 390-450 mg Theobromine per oz chocolate
Milk chocolate = 44-60 mg Theobromine per oz chocolate
Semi-sweet is a bit less than half of the Theobromine content in Baker’s chocolate.
How much is too much?
The toxic dose of Theobromine (and caffeine) for pets is 100-200mg/kg. (1 kiliogram = 2.2 pounds). However, various reports by the ASPCA (American Society for the Prevention of Cruelty to Animals) have noted problems at doses much lower than this – i.e. 20mg/kg.
http://www.msdvetmanual.com/toxicology/food-hazards/chocolate