Bhashya
अन्धम् अदर्शनात्मकं तमः संसारनियामकं प्रविशन्ति प्रतिपद्यन्ते ; के ? ये अविद्यां विद्यातोऽन्यां साध्यसाधनलक्षणाम्, उपासते, कर्म अनुवर्तन्त इत्यर्थः ; ततः तस्मादपि भूय इव बहुतरमिव तमः प्रविशन्ति ; के ? ये उ विद्यायाम् अविद्यावस्तुप्रतिपादिकायां कर्मार्थायां त्रय्यामेव विद्यायाम्, रता अभिरताः ; विधिप्रतिषेधपर एव वेदः, नान्योऽस्ति- इति, उपनिषदर्थानपेक्षिण इत्यर्थः ॥
In plain words
'Blind' means simply not seeing; 'darkness' is whatever keeps the wheel of birth and death turning; and 'entering' means undergoing that darkness. Who does this? Those who take up ignorance — which is nothing but the means to some result, entirely apart from real knowledge — and pursue it, meaning they just keep performing ritual action. That is the meaning. Now, a darkness even greater than that one is entered by someone else. Who? Those who bury themselves in a so-called 'knowledge' that only explains ritual matters, whose whole purpose is action — nothing more than the triple Vedic lore — and who become utterly absorbed in it, thinking 'the Veda is only about what to do and what not to do, there is nothing beyond that.' In doing so they turn away entirely from what the Upanishads mean. That is the sense.
Close to the Sanskrit
'Blind' means characterized by non-seeing; 'darkness' means that which governs transmigration; 'they enter' means they undergo. Who? Those who, ignorance being different from knowledge and characterized as the means to an end to be accomplished, 'worship' it, that is, they keep following that ritual action — that is the meaning. Than that, a darkness greater still, as it were, they enter. Who? Those who, in the knowledge that only expounds the matter of ignorance, whose purpose is ritual action, that is, only the triple Vedic knowledge, are devoted, that is, utterly absorbed — [holding] 'the Veda is concerned only with injunction and prohibition, there is nothing else' — thus they disregard the meaning of the Upanishads; that is the meaning.