Dominant genes will come before recessive. Someone with brown-eye and blue-eye genes will have brown eye since it's the dominant one. So sometimes blue -eye allele is hidden because it's not express, but not always.
People with blue eyes must have all blue-eye allele. Therefore, if there are people with blue eyes you can conclude that blue-eye allele has not disappeared in the human population.
If there is no factor that intervenes with population breeding, Hardy-Weinberg equilibrium will be maintained and that can be used to explain why the all the blue-eye alleles have not disappeared in the human population