[xkcd-sucks]

With ferrocyanide in particular ([Fe(CN)6]4−), the reason cyanide (CN-) is such bad news is also the reason it's safe complexed to iron: Iron-Cyanide coordination is very strongly favored by thermodynamics and kinetics, so it likes to stick to iron, sticks very quickly, and un-sticks itself very slowly. When cyanide is not already coordinated to iron, it coordinates to the iron in hemoglobin (+ COX enzymes in mitochondria, and other less immediately important enzymes) and makes them not work, basically forever. But when CN- is already stuck coordinated to free iron, it doesn't really fall off enough to stick itself to enzymes in any meaningful amount. It would be more of a problem if CN- accumulated, but it's metabolized and excreted quickly enough to not be a big deal. The LD50 values basically reflect this.