Albert Barnes on James 3:10

Out of the same mouth proceedeth blessing and cursing. The meaning here may be, either that out of the mouth of man two such opposite things proceed, not referring to the same individual, but to different persons; or, out of the mouth of the same individual. Both of these are true; and both are equally incongruous and wrong. No organ should be devoted to uses so unlike, and the mouth should be employed in giving utterance only to that which is just, benevolent, and good. It is true, however, that the mouth is devoted to these opposite employments; and that while one part of the race employ it purposes of praise, the other employ it in uttering maledictions. It is also true of many individuals that at one time they praise their Maker, and then, with the sane organ, calumniate, and slander, and revile their fellow-men. After an act of solemn devotion in the house of God, the professed worshipper goes forth with the feelings of malice in his heart, and the language of slander, detraction, or even blasphemy on his lips.

My brethren, these things ought not so to be. They are as incongruous as it would be for the same fountain to send forth both salt water and fresh; or for the same tree to bear different kinds of fruit.