What I really need is a 4 x 4 - 6 x 6ish dark red or burgundy flower plant suggestion with all green leaves that will be located in between two blue hydrangeas. I am hoping that they will all grow into each other when they become large enough to hide the backyard fence. It will be a centerpiece plant between two blue hydrengeas and I was really trying to avoid anything too pink in this part of the yard. I just found a wonderful little red dwarf oleander that would be perfect because the flowers are darker than true red, but they are poisonous, so I am still looking. I am hoping to find something that will flower frequently. I am desperately needing some suggestions please!! Thanks!
The oleander wouldn't get along well with the hydrangeas anyway--oleanders do better in sun & a drier location, vs hydrangeas like shade and more water. I had an oleander in dry shade at my old house and it didn't bloom very well. Wetter shade would probably make it even unhappier.
For burgundy flowers you might consider Calycanthus http://davesgarden.com/guides/pf/go/59489/ It likes shade and should do fine there. You could probably also find an azalea with a flower color you liked--some of them will be too pink but there are so many cultivars I bet you could find one you like.
If you'd take burgundy leaves instead of flowers, you might consider one of the darker leaf Loropetalum cultivars such as 'Burgundy' http://davesgarden.com/guides/pf/go/56038/ The flowers are pink, but they're small and not overpowering and it'll bloom earlier than the hydrangeas so it won't be competing with them.
How about an Azalea bush with the hydrangea. I have that combination and it beautiful. The Azalea blooms in late spring and the hydrangea bloom through summer and the blooms changes color in fall. Hope this can help
This isn't exactly a burgundy suggestion but how about a California bush poppy: http://davesgarden.com/guides/pf/go/175247/
They meet the size requirement and have gorgeous white "poppy" flowers. I bet that would look nice with those hydrangeas.