There aren't many bulbs that would be happy with wet feet. However, calla lilies can handle damp conditions, as can Onixotis/Dipidax. Some of the iris species will grow well in periodically damp/wet soil and could give you the planting height you might be after.

Otherwise - sedges and rushes, preferably native, which will help with filtering any pollutants you might be getting from the runoff.

Sooner or later (said in a Warning Tone) you will probably have to liberate them from the confines of the cinder blocks. OTOH - planting in front of the blocks will let you use them as hard landscaping softening by appealing greenery ;-))

Most glads would be happier in drier conditions, in my view.

Good luck with the rain garden; I think it is a practical and civil thing to do to abate run-off and pollution. Thanks.

possibly G.flanaganii? Not too genned up on glads but there are many which will grow with wet feet but they also need heat (south Africans). try the pacific bulb website.

Thanks for your responses!

If I dig the rain garden deeper than the cinder blocks I'm planting the Gladiolus corns in, to protect them from getting soaked, do you think the narrowness of the cinder blocks themselves will present a problem?

My runnoff area is about 200 sq feet (I have 3 drain spout on the back of the house and the house is small). The rain garden will be about 50 sq. feet. The blocks will be protected by a four inch depth of dirt around the outside (leaving four inches exposed)and will border the rain garden about two inches about the garden's soil depth. It's basically a hole with a shelf around the border that the cinder blocks will sit on. The corns will be inside the blocks, about an inch above the ground level of the blocks/7 inches deep(3 inches above the rain garden's soil level).