If we Catholics were driven underground what could the priest use as an alternative for the Blood of Christ, I know that we only need the Body of Christ but hypothetically could we use Port, Sherry, something else or does it have to be normal wine?
For the Distribution that is true, but for a valid Mass the Precious Blood has to be consecrated, even if only consumed by the priest.
As to the main question, Canon Law (Can. 924 §3) just says the wine must be “natural, from grapes of the vine, and not corrupt.” The 16th c. document De Defectibus, IV, 11 says “If the wine has become mere vinegar, or is completely bad, or if it has been made from sour or unripe grapes, or if so much water has been mixed with it that the wine is adulterated, there is no Sacrament.”
If you don’t have any matter to use (grape wine + wheat) then there’s just no Mass. You have prayer services and a priest can be around for Reconciliation and Healing (if you have the oil). Wine has been a difficulty on and off in Islamic nations, though not really these days. Drinking the Precious Blood as a Catholic is analogous to a grave sin in Islam. You also had Mass outlawed in Europe for a time in some places in the 1500s, and of course, on and off during pagan Rome. You just keep doing it off-the-books.
Some wines have names that happen to contain the words “port” or “sherry” but are still valid matter. The name, all by itself, isn’t always an indicator. For example, here’s a link to an approved altar wine called “Port.” montlasallealtarwines.com/wines.php#wine6
The typical “port” that would be sold in a wine shop would not be valid matter.
The juice of unripe grapes is at the stage of incomplete generation, and therefore it has not yet the species of wine: on which account it may not be used for this sacrament. Must, however, has already the species of wine, for its sweetness indicates fermentation which is the result of its natural heat; consequently this sacrament can be made from must. (III:74:5)
He goes on to warn, however, that the direct pouring of juice pressed from grapes into a chalice is prohibited unless truly necessary.
Agreeing with Aquinas’ reasoning, the New Commentary on the Code of Canon Law asserts: “Canonists and theologians have commonly held that ** mustum, or the unfermented juice of ripe grapes, is valid matter for the Eucharist but is gravely illicit except in necessity**.” When is it necessary and therefore licit (or allowed)? The CDF document Norms for Use of Low-Gluten Bread and Mustum stipulates that “the permission to use mustum can be granted by ordinaries to priests affected by alcoholism or other conditions which prevent the ingestion of even the smallest quantity of alcohol, after presentation of a medical certificate” (II:B).