Preferential access for blind users?

There are a wide variety of vision impairments that a user might have, and knowing whether a user is specifically blind is not within the scope of the Accessibility frameworks. If you really want to limit it to just blind users, you may need to have some way of marking user accounts for special access, and then invite such users to contact you directly to get their account marked accordingly.

However, you can check whether a user has certain accessibility. For example, you could check UIAcessibility.isVoiceOverRunning to see if the user has VoiceOver enabled, and adjust the features in your app accordingly.

Be aware that many blind users likely have VoiceOver enabled, but other vision-impaired-but-not-completely-blind users may also enable VoiceOver. Further, any user can enable VoiceOver (even non-blind users), so if you gate access to features in your app based on whether VoiceOver is active, be aware that some non-vision-impaired users may use this to gain access to that feature.