Short answer: It’s best to avoid black coffee during Paryushan.

Traditional guidance (both traditions)

• On Upvās, Attham, Atthai, etc.: Only boiled water (previously boiled and cooled) within fixed daylight hours is allowed. Tea/coffee—black or with milk—are not permitted.
• On Ekāsan/Biyasana (one or two meals): Take simple, non‑stimulating food before sunset. Tea/coffee are generally renounced as part of ras-tyāg (giving up stimulating tastes) and for saiyam (self‑restraint).

Why it’s avoided

• Ras‑tyāg: Coffee is a stimulant and increases raga (attachments).
• Niyam: During Paryushan we simplify aahar (diet) to reduce new karma.
• Ahimsā/saṃyama: Repeated preparations and addictions are discouraged; boiled water rules keep intake minimal and careful.

Tradition notes

• Shvetāmbara Paryushan: Standard practice is boiled water only on fasting days; tea/coffee are not taken.
• Digambara Das‑Lakshan: Disciplines are the same or stricter; tea/coffee are not taken on vrata days.

If you have a medical need, take guidance from your guru/pravartini/ācārya and keep the intent of pramāda‑tyāg (avoiding carelessness). If any relaxation is taken for health, do it within daylight, as simply as possible, and with pratikraman-bhāva.