Answer: The Gemara (Taanis 26b) forbids eating meat and drinking wine at the seuda hamafsekes, though many rishonim record the minhag to abstain throughout this time period. Thus, the Shulchan Aruch (OC 551:9) writes that there are various minhagim as to exactly when this applies. Some just refrain throughout the week of Tisha B’av, others from Rosh Chodesh Av while others refrain throughout the entire three weeks. The Mishna Berura (551:58) writes that the ashkenazi minhag is to refrain from meat and wine during the nine days.
The Aruch Hashulchan (OC 551:23) explains that this minhag serves to remind us of the korbanos that we can no longer offer up since the churban (See Tur OC 552:2; Biur Hagra OC 551:11). While it is a minhag, he notes that it is observed universally and so one who eats meat has broken a communal vow which is a Torah prohibition (See Mordechai, Taanis 639).
On the other hand, the Gemara (Berachos 33a) writes that one who recites an unnecessary beracha has transgressed the Torah prohibition of lo sisa.
R’ Ovadia Yosef (Yabia Omer YD 2:5:11) and R’ Ephraim Greenblatt (Rivevos Ephraim 3:378) quote the Sdei Chemed (Bein Hametzarim 1:5) who writes that if one accidentally recited the beracha on meat they should take a small bite. Doing so ensures that one hasn’t said a beracha levatala, and being so small ensures that one does not receive any simcha, especially as they are primarily doing so to avoid saying a beracha levatala.
In conclusion, if one accidentally said a beracha on meat during the nine days they should eat a tiny piece.