It was a disappointing result after a spirited performance by a depleted Ivybridge side.
They went into the game without Connor Rush and Owen Raven, who were both cup-tied, while full-backs Curtis Legrice and Jake Lane were ruled out through injury.
Despite those problems Ivybridge made a lively start and took the lead in the 13th minute when CJ Pritchard scored with a long-range effort from out on the left that curled into the far corner of the net.
St Blazey equalised, however, with a well-placed shot from Liam Eddy in the 33rd minute.
Ivybridge created a number of opportunities to have regained the lead in the second half but lacked the necessary finishing touch and were made to pay in the first of six added minutes when Eddy headed the winner, with his second goal of the game, following a right-wing corner.
St Blazey will now meet Torpoint in the final at Launceston on May 7.
Meanwhile, Ivybridge return to South West Peninsula League action on Saturday when they visit troubled Ottery St Mary, who are on a run of nine successive defeats. Ivybridge will be looking to improve on a run that has brought only two wins in the last nine league matches.