What makes a baby girl have blonde hair and blue eyes?

It's about the genes. Imagine the colour of the hair is determined by a gene. This gene (hair colour) has different variations (black, brown, blond...). Some colours are more likely to be seen.  

There is the difference between the genotype and the phenotype.

Genotype is related to the genes, the material we have in our DNA.

Phenotype is the expression of the genes, it's what we see (the hair colour in this case).

In each person, the gene has a couple of alleles. It takes the form AA.

Let's suppose A is the letter for the black hair, B for the blond hair. If we had a person with genotype as AB, the phenotype would be dark hair, as it prevalences against light colours. (Dark hair is a dominant allele, while blond hair is a recessive allele). Even though, that person would carry the allele for the blond hair.

In the case of your question, the genotypes would be;

Mother:: dark hair, so she has the allele A

Father:: dark hair, so he also carries the allele A.

However, dark hair doesn't avoid the existance of the allele for blonde hair; so both parents may have the allele (blond hair) which isn't shown because it's recessive.  

To sum up, it's possible because both parents have the genotype AB, so the phenotype is dark hair for both though they both carry the allele for the blond hair.

When reproducing, the genes of both parents get mixed, so the possible combinations for the alleles would be:

AA   AB     BB

In this case, the baby has blond hair, so he/she must carry both of the recessive alleles, so the genotype is BB.  

In relation to the eyes  it would be the same.

Explanation:

a tetrad is the foursome during meiosis made by two homologous chromosomes that have each already replicated into a pair of sister chromatids.