May one recite Birchas Ilanos over blossoming or ripening citrus fruit trees, such as a lemon tree or orange tree? Someone who has knowledge in agronomy told me that all citrus fruits today are grafted and hence we cannot say the blessing over it. Is this true?

You may recite a blessing over it, unless you see vivid evidence on the tree that it was grafted with another species of citrus fruit.

Explanation: It is debated whether a blessing may be said over a grafted tree if it was grafted in a forbidden way with a different species. Now, although practically we rule that one should not initially say the blessing over a prohibited grafted tree, nonetheless, this only applies if one is for certain that it was grafted in a forbidden way. However, if this is a mere doubt, then it is allowed. Accordingly, there is no problem with saying the blessing over a citrus tree as even though citrus trees are, being that a) it is possible that it was grafted with a branch of the same species. B) it is only possible to graft citrus fruit with another citrus fruit branch and according to some Poskim all citrus fruits are considered of the same species regarding the laws of grafting and it is hence permitted. Hence, there is a double doubt, known as a Sfek Sfeika, if there is any grafting prohibition in the citrus tree, and hence you may say the blessing over it even initially.

Sources: See Reb Akivah Eiger 226; Divrei Malkiel 3:2; Rav Poalim 2:36; Kaf Hachaim 226:11; Yagel Yaakov 12 in name of Chasam Sofer 6:25; Minchas Yitzchak 3:25; Yabia Omer 5:20; Piskeiy Teshuvos 226:6; Nitei Gavriel 6:13 footnote 16