I cite from the official FAQ of
https://www.catan.com/faq/basegame
Question:
Seven and Robber - After rolling a “7,” can I still trade with another player or blackmail him before moving the robber?
Answer:
*) Trading: No.
*) Blackmailing: Yes. On Catan, people are allowed to talk to each other at any time. Therefore, even if it is not their trade and build phase, all players are free to make offers for future bargains, and if they see fit, they may also blackmail and threaten other players. However, all this can only be put into action after the consequences of the dice roll are resolved. You can’t perform trades to reduce the number of your resources to seven or fewer before resolving the consequences of the “7.”
Personal comment:
Thus, after a 7 is rolled, the blackmailing of "Orange will you trade me wood? I will rob Blue if you are willing to trade a wood." is allowed.
Of course Orange could accept the deal via chat and after Blue is robbed he may change his mind. Orange is not obliged to keep his promise because he is allowed to lie.
Both the original blackmailing and/or the non-fulfilling of Orange's promise can (and probably will) cause some bad emotions...