The answer to the question may depend on a question of how ikar and tafel works:
The standard approach is that the blessing on the primary covers the secondary. You make the blessing on the main food and it extends to the other food. This may be because the other food is seen as having become conceptually part of the main food, or nullified to it for berachos purposes. This is the simple understanding, and many poskim are explicit like this (Chazon Ish and others). According to this, if the tafel was not there and you did not have intention to eat it when you made the blessing, then it wouldn't be covered.
However, there is another approach, which is that a food that is secondary is not significant enough to require a blessing at all (Rashba citing the Ba'al HaMeor in Berachos 41b s.v. Amar Rav Papa)! According to this approach, the blessing on the primary is not covering the secondary, but rather the secondary does not need a blessing because it is not a significant part of this meal. If that is the case, your tafel would still not need a blessing, because it is still a tafel and not significant enough to require a blessing.
Normative halacha seems to follow the first approach, although there are arguments to be made for the second. (Of course, CYLOR.)
If you had it in mind, then even according to the first approach it could be you would not need a new blessing, since the blessing on the ikar would extend to the tafel.