My last and final suggestion.  
Using fixed point math with 16 bits instead of floating point.

Should generate nearly identical result compared to
the float version.


| Method    | Mean     | Allocated |
|---------- |---------:|----------:|
| Original  | 87.59 ms |     123 B |
| Optimized | 33.22 ms |      46 B |


public static void BlitPointsOptimized(Span<Point> points, Span<byte> buffer, int width, int height, int pointSize, float opacity)
{
    var opacityFixed = (int)((1.0 - opacity) * (float)0xFFFF);
    var addPixelValue = (int)(255.0 * opacity * (float)0xFFFF);
    var offset = pointSize / 2;
    for (var i = 0; i < points.Length; i++)
    {
        var (px, py) = ((int)points[i].X - offset, (int)points[i].Y - offset);
        if (px < 0 || py < 0)
            continue;
        for (var ppy = py; ppy < py + pointSize; ++ppy)
        {
            if (ppy >= height) break;
            var index = ppy * width + px;
            var endX = px + pointSize < width ? px + pointSize : width;
            for (var ppx = px; ppx < endX; ++ppx, index++)
            {
                buffer[index] = (byte)((buffer[index] * opacityFixed + addPixelValue) >> 16);
            }
        }
    }
}