andha tāmasa

 The standard Devanagari spelling that corresponds to andha tāmasa is अन्धतामस.

What the dictionaries show

The Sanskrit dictionary entry for tāmasa gives the Devanagari form तामस

Sanskrit Dictionary

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The compound andhatamasa / andha‑tāmasa appears in multiple dictionary listings with the Devanagari spellings अन्धतमस and अन्धतामस

Wisdom Library

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Matching the user’s transliteration

Your phrase is written with a space: andha tāmasa.
This corresponds to two separate words:

• andha → अन्ध
• tāmasa → तामस

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Putting them together as written (not compounded) yields:

अन्ध तामस

If you intend the compound (as in most Sanskrit usage)

Most Sanskrit sources treat this as a karmadhāraya compound meaning “utter darkness.”
The standard compound spellings attested are:

• अन्धतमस (andhatamasa)

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• अन्धतामस (andhatāmasa)

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  1 Source

Of these, अन्धतामस most closely matches your transliteration andha tāmasa because it preserves the long vowel ā of tāmasa.

Summary

• Word‑for‑word spelling: अन्ध तामस
• Standard compound spelling matching your transliteration: अन्धतामस
• Alternative attested compound: अन्धतमस