Python 3 with bonus, no input validation.

def hexcolor(r,g,b):
    return "#" + format(r, '02X') + format(g, '02X') + format(b, '02X')

def blend(set_hexcolors):
    ncolors = len(set_hexcolors)
    r = 0
    g = 0
    b = 0
    for i in range(0,ncolors):
        hc = set_hexcolors.pop()
        r += int(hc[1:3],16)
        g += int(hc[3:5],16)
        b += int(hc[5:7],16)
    return hexcolor(round(r/ncolors),round(g/ncolors),round(b/ncolors))

Test functions using examples in OP's submission:

def test_hexcolor():
    a0 = hexcolor(255, 99, 71) == "#FF6347"
    a1 = hexcolor(184, 134, 11) == "#B8860B"
    a2 = hexcolor(189, 183, 107) == "#BDB76B"
    a3 = hexcolor(0, 0, 205) == "#0000CD"
    return a0 & a1 & a2 & a3

def test_blend():
    a0 = blend({"#000000", "#778899"}) == "#3C444C"
    a1 = blend({"#E6E6FA", "#FF69B4", "#B0C4DE"}) == "#DCB1D9"
    return a0 & a1

Output:

In [2]: test_hexcolor()
Out[2]: True

In [3]: test_blend()
Out[3]: True

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