I assume that you will be planting established plants from a nursery? If that is the case, go ahead and plant them now. Just mulch around the plants, but not up against the stem, to protect them from our excessive heat which has hit the 100-degree mark the last few days in Houston, and give them enough water until well established.
Keep in mind that black-eyed Susan's like to grow in full sun. There are different varieties of them. Some are perennial, and some are annuals. They can grow to different heights. Make sure that you know which type you wish to plant so that you give them enough space in your garden.