Here’s a trick for choosing tomatoes that’ll do well in cool summers: Look in the names or descriptions for those that indicate Siberia, Russia, or other cold locations.
What you ultimately want is a short-season tomato that ripens in 70 days or less. Smaller varieties (like cherry or salad tomatoes) and determinate varieties (bushy plants that set all their fruits in a concentrated period of time) also have a better chance of ripening before frost.
The most important thing to remember is that the time to ripen (also known as days to maturity or days to harvest) is expressed as number of days after transplanting outdoors.
So even though Black Prince should ripen in 70 days (give or take a few), it could take up to 84 days or even longer, depending on day length and weather in your area.
Your best bet is to err on the side of being “too early” if you’re trying to beat the clock on harvesting before frost.