You may be required to wear certain things like comfortable shoes/coats/jackets if your excursion is in any way physical
But where persons wear mourning for style and not for feeling, there is a need of fixed rules. Interestingly, Helena Rowes article, Family Fashions and Fancies: Mourning CostumesEnglish and French Styles, featured in Good Housekeeping, Volume 9 (1889), makes a note of the inability to lay down rules while laying down a rule at the same time: The London code prevails in New York in all these matters
Add some black jewelry or even a black ribbon as shown here
Look for a style with a square neckline, tie-shoulder straps, or a smocked back for comfort and breathability
Taller ladies will want to hide their knobby knees, while shorter ladies dont want all that fabric around their ankles
Style cohesion between dress and shoes becomes crucial with more visibility