This is a “bug” in older versions of matplotlib (and has been fixed for the 1.4 series). The issue is that in Axes.errorbar there is a default value of '-' for fmt, which is then passed to the call to plot which is used to draw the markers and line. Because a format string is passed into plot in never looks at the default value in rcparams.
You can also pass in fmt=""
eb = plt.errorbar(x, y, yerr=.1, fmt="", color="b")
which will cause the rcParam['lines.linestlye'] value to be respected. I have submitted a PRto implement this.
Another work around for this is to make the errorbar in two steps:
l0, = plt.plot(x,y, marker="o", color="b")
eb = plt.errorbar(x, y, yerr=.1, fmt=None, color="b")
This is an annoying design decision, but changing it would be a major api break. Please open an issue on github about this.
errorbar doc.
As a side note, it looks like the call signature was last changed in 2007, and that was to make errorbars not default to blue.