The default colour cycle was changed in matplotlib version 2 as shown in the docs.
Therefore, to plot the “new” default blue you can do 2 things:
fig, ax = plt.subplots()
ax.scatter(-1, 1)
ax.text(-0.9, 1, 'Default blue')
ax.scatter(1, 1, c="#1f77b4")
ax.text(1.1, 1, 'Using hex value')
ax.scatter(0, 0.5, c="C0")
ax.text(0.1, 0.5, 'Using "C0" notation')
ax.set_xlim(-2, 3)
ax.set_ylim(-1,2)
plt.show()
Which gives:
Alternatively you can change the colour cycle back to what it was:
import matplotlib as mpl
from cycler import cycler
mpl.rcParams['axes.prop_cycle'] = cycler(color="bgrcmyk")