The genetics of hair color aren't fully understood, but the production of two different compounds determines hair color: melanin and eumelanin.
Lets build a punnet square for two traits. Assume Bob is Homozygously recessive for melanin production and Heterozygous for eumelanin (blonde hair). We'll say Helen is a reddish brunette, it's actually hard to say exactly what her phenotype is, and even harder to get the genotype. However, we'll assume she is Heterozygous for melanin and Homozygous recessive for eumelanin (reddish brown).
This could easily produce a child who is like Bob, Homozygous recessive for melanin and heterozygous for eumelanin. You could also get a heterozygous individual for both melanin and eumelanin. This isn't exactly what makes raven hair, but without getting into more complex genetics, it produces a plausible groundwork. This particular punnet square would also open the possibility for Jack Jack to have red hair, though it is likely it will darken to match Helen's hair as he grows up. You would expect mostly brown haired kids from Bob and Helen, but the phenotypes of the children are not impossible.