The blue color is not defied by us. It is defined in your theme CSS file:
https://www.badkamerenkeukencursus.nl/wp-content/themes/blocksy/static/bundle/main.min.css?ver=1.8.4.5
Here:
select, textarea, input[type='url'], input[type='tel'], input[type='text'], input[type='date'], input[type='email'], input[type='number'], input[type='search'], input[type='password'], .ct-widget select, fieldset .wc-stripe-elements-field {
-webkit-appearance: none;
appearance: none;
width: var(--form-field-width, 100%);
height: var(--form-field-height, 40px);
font-family: inherit;
font-size: var(--form-font-size, 15px);
color: var(--form-text-initial-color, var(--color));
padding: var(--has-classic-forms, var(--form-field-padding, 0 12px));
border-width: var(--form-field-border-width, 1px);
border-style: var(--form-field-border-style, solid);
border-color: var(--form-field-border-initial-color);
border-radius: var(--has-classic-forms, var(--form-field-border-radius, 3px));
background-color: var(--has-classic-forms, var(--form-field-initial-background));
transition: all 0.12s cubic-bezier(0.455, 0.03, 0.515, 0.955);
You can try to override it in Appearance -> Customize -> Additional CSS:
textarea {
color: white !important;
}