The labels were changed from blue to purple on 3.4 for accessibility reasons but they are in blue on 3.5 and master.

This was discovered while testing MDL-62660 , and .MDL-59448 was cited as related.

See the note on https://getbootstrap.com/docs/4.0/getting-started/accessibility/#color-contrast

Color contrast
Most colors that currently make up Bootstrap’s default palette—used throughout the framework for things such as button variations, alert variations, form validation indicators—lead to insufficient color contrast (below the recommended WCAG 2.0 color contrast ratio of 4.5:1) when used against a light background. Authors will need to manually modify/extend these default colors to ensure adequate color contrast ratios.