As senior pranks go, this is a pretty creative one!! And if done well, one that could be much appreciated for sometime.

Bulbs may be your best bet - they are pretty much self-contained plants and require no special attention after planting. But planting them among brush may pose some problems. What is the nature of the brush? Blackberries? Ivy? Bracken fern? All of these and many other common brushy weeds are pretty vigorous plants and have the ability to grow back rapidly and obscure the intended planting, if not smother it outright. Some serious clearing may have to be done.

Blue poppies (Meconopsis) are woodland plants and require very specialized conditions to thrive. They are also very expensive to purchase as starts and difficult to grow from seed sown directly in the soil. And unfortunately, they are not the slightest bit invasive - far from it!! They are kind of the holy grail of NW gardeners and truly a poor choice for this situation.

But there ARE blue flowering bulbs - hyacinths and wood hyacinths or bluebells (Hyacinthoides/Scilla) are two that come to mind; also iris, chionodoxa, grape hyacinths (muscari) and even some 'blue' (sort of) tulips. The scilla would make a good combo with the daffs and both have the potential for naturalizing or returning for years. But a simultaneous bloom time is going to be hard to achieve - most narcissus (daffodils) tend to be early spring bloomers while most of the other blue flowering bulbs will be mid to late spring bloomers.

Other possibilities may be forget-me-nots, which are easy to start from seed and self-sow abundantly and can bloom very early if in sufficient sun. You could sow the seeds of those in fall when you plant the white bulbs for hopefully a simultaneous flower display.

If you go this way, some "blue" tulips to look for are 'Blue Amiable', 'Blue Diamond' or 'Blue Lagoon'. Too late now to order these for fall planting but you might be able to locate them at better nurseries that carry an extensive bulb selection. The other blue flowering bulbs should not be difficult to locate.

Some good points were brought up about determining what the existing brush is before you go planting. Even if you clear it all away now, it will probably require some additional clearing/cutting back at least monthly to keep the slope clear enough for low growing bulbs to be visible. The For-get-me-nots are probably a great suggestion, and the white daffodils can be guaranteed to bloom concurently, and as long as they get sun and are not overgrown by taller brush, they are pretty hardy, as deer don't eat them nor do gophers. If you have heavy slug/snail populations, however, they can be mowed down.

Other plants that could work well for a blue and white combination might include other annuals/perennials that selfsow or grow with little care. Centranthus ruber 'Albus' is extemely easy, and could be combined with Blue Anemones. The bloom season might not be earlier enough for what you have in mind, however. Another combination could be White Alyssum along with Ipheion uniflora, but as these are both very short growers, they could easily be overwhelmed by brushy weeds.

Crocuses come in blue and white, would bloom at the same time and are quite cheap so you could make a big display without spending very much. Good luck with your project.