If I do
echo -e '\033[31m\033[5mHello\033[0m'
I get red (\033[31m) blinking (\033[5m) text both in XTerm and URxvt.
However, if I enter
echo -e '\033[41m\033[5mHello\033[0m'
which should set the red background (\033[41m) (instead of foreground \033[31m) under a blinking (\033[5m) text, I see it working as expected in XTerm, but the blinking is off in URxvt:

Swapping the two escape sequences has no effect.
Is this a bug or "feature" of URxvt? Or how can I make the text blink when a background is specified?