In short: yes, blue tongue skinks can eat apples and it is perfectly acceptable to feed them these fruits as part of a balanced diet.
Apples are high in fructose and their consumption should be limited, due to their high sugar content which may lead to obesity. Moreover, you need to make sure to remove any apple seeds before serving because they are hard to digest and may lead to impaction.
Other than that, apple is a healthy fruit choice and you can feed it to your blue tongued skink in moderation without any major concerns.
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