A good improvement in the RadHtmlChart would be to add an Opacity property for the series, which would allow the control of its transparency. The property can be set through the chartObject:
JavaScript:
		<script>
			function pageLoad() {
				var chart = $find("<%=BubbleChart.ClientID%>");
				chart._chartObject.options.series[0].opacity = 0.3;
				chart.repaint();
			}
		</script>
ASPX:
		<telerik:RadHtmlChart runat="server" ID="BubbleChart" Width="500" Height="400">
			<PlotArea>
				<Series>
					<telerik:BubbleSeries>
						<Appearance FillStyle-BackgroundColor="#6ab2c9">
						</Appearance>
						<SeriesItems>
							<telerik:BubbleSeriesItem Size="3" X="5" Y="5500" />
							<telerik:BubbleSeriesItem Size="12" X="14" Y="12200" />
							<telerik:BubbleSeriesItem Size="33" X="20" Y="39000" />
							<telerik:BubbleSeriesItem Size="10" X="18" Y="24400" />
							<telerik:BubbleSeriesItem Size="42" X="20" Y="32000" />
						</SeriesItems>
					</telerik:BubbleSeries>
				</Series>
			</PlotArea>
		</telerik:RadHtmlChart>