Meat should be slightly resilient and moist. Improper storage temperatures, refreezing or extended frozen storage can turn shrimp tough, dry and fibrous. A black spot on the shell indicates melanosis. It's not a health issue in early stages but indicates general poor quality. Just because black tigers are farmed doesn't mean they haven't been treated.
Some countries routinely treat tigers with sulfites to prevent melanosis.
Pitted shells and a soapy flavor indicate oversoaking. Chem-free shrimp is available, but be prepared to pay a premium. They're good with pasta or in casseroles, since they can withstand tossing with other ingredients. Their thick shells hold heat, so black tigers cook more quickly than the other Penaeids. For maximum quality, eat tiger shrimp within two days of thawing, though they will last three to four days properly refrigerated.