Praise be to Allah, the Lord of the Worlds.
Sending blessings upon the Prophet (Blessings and peace be upon him). when the Khateeb sends blessings upon him or mentions his name isn`t part of the prohibited speech during Jumu`h. What is prohibited is whatever speech hinders listening to the Khateeb or implies that the praying person/listener isn`t actually listening to him. As for sending blessings upon the Prophet (Blessings and peace be upon him) whenever the Khateeb sends blessings upon him or mentions his name, that is allowed and there is no evidence from the texts of Sharia indicating otherwise. Rather, many unconditional texts indicated that it is recommended to send blessings upon the Prophet (Blessings and peace be upon him). Also, there are many authentic Hadiths in support of this, such as the Hadith where Anas Bin Malik narrated that one of the nomads asked the Prophet (Blessings and peace be upon him) to invoke Allah for rain because there was a drought. Also, the Hadith of Solaik Al-Ghaftani whom the Prophet (Blessings and peace be upon him) ordered to perform two Rakahs before sitting down after entering the mosque. This is in addition to many other authentic Hadiths.
The prominent scholar, Ar-Ramli, said: "During the Jumu`ah sermon, it is allowed for the listeners to send blessings and peace upon the Prophet (Blessings and peace be upon him) loudly whenever the Khateeb mentions his name or sends blessings upon him." {Nehayat al-Mohtajj, vol2/pp.320}. And allah Knows Best.