The design is the shape and configuration of the Blade for a Sickle shown in the drawings.Drawings of the design are included as follows:Figure 1 is an isometric view from the top and one side of the Blade for a Sickle showing the design.Figure 2 is a first side elevational view of the Blade for a Sickle showing the design.Figure 3 is a second side elevational view of the Blade for a Sickle showing the design.Figure 4 is a front elevational view of the Blade for a Sickle showing the design.Figure 5 is a rear elevational view of the Blade for a Sickle showing the design.Figure 6 is a top plan view of the Blade for a Sickle showing the design.Figure 7 is a bottom plan view of the Blade for a Sickle showing the design.Figure 8 is an isometric view from the top and one side of the Blade for a Sickle showing a first variant of the design.Figure 9 is a first side elevational view of the Blade for a Sickle showing a first variant of the design.Figure 10 is a second side elevational view of the Blade for a Sickle showing a first variant of the design.Figure 11 is a front elevational view of the Blade for a Sickle showing a first variant of the design.Figure 12 is a rear elevational view of the Blade for a Sickle showing a first variant of the design.Figure 13 is a top plan view of the Blade for a Sickle showing a first variant of the design.Figure 14 is a bottom plan view of the Blade for a Sickle showing a first variant of the design.Figure 15 is an isometric view from the top and one side of the Blade for a Sickle showing a second variant of the design.Figure 16 is a first side elevational view of the Blade for a Sickle showing a second variant of the design.Figure 17 is a second side elevational view of the Blade for a Sickle showing a second variant of the design.Figure 18 is a front elevational view of the Blade for a Sickle showing a second variant of the design.Figure 19 is a rear elevational view of the Blade for a Sickle showing a second variant of the design.Figure 20 is a top plan view of the Blade for a Sickle showing a second variant of the design.Figure 21 is a bottom plan view of the Blade for a Sickle showing a second variant of the design.Figure 22 is an isometric view from the top and one side of the Blade for a Sickle showing a third variant of the design.Figure 23 is a first side elevational view of the Blade for a Sickle showing a third variant of the design.Figure 24 is a second side elevational view of the Blade for a Sickle showing a third variant of the design.Figure 25 is a front elevational view of the Blade for a Sickle showing a third variant of the design.Figure 26 is a rear elevational view of the Blade for a Sickle showing a third variant of the design.Figure 27 is a top plan view of the Blade for a Sickle showing a third variant of the design.Figure 28 is a bottom plan view of the Blade for a Sickle showing a third variant of the design.Figure 29 is an isometric view from the top and one side of the Blade for a Sickle showing a fourth variant of the design.Figure 30 is a first side elevational view of the Blade for a Sickle showing a fourth variant of the design.Figure 31 is a second side elevational view of the Blade for a Sickle showing a fourth variant of the design.Figure 32 is a front elevational view of the Blade for a Sickle showing a fourth variant of the design.Figure 33 is a rear elevational view of the Blade for a Sickle showing a fourth variant of the design.Figure 34 is a top plan view of the Blade for a Sickle showing a fourth variant of the design.Figure 35 is a bottom plan view of the Blade for a Sickle showing a fourth variant of the design.Figure 36 is an isometric view from the top and one side of the Blade for a Sickle showing a fifth variant of the design.Figure 37 is a first side elevational view of the Blade for a Sickle showing a fifth variant of the design.Figure 38 is a second side elevational view of the Blade for a Sickle showing a fifth variant of the design.Figure 39 is a front elevational view of the Blade for a Sickle showing a fifth variant of the design.Figure 40 is a rear elevational view of the Blade for a Sickle showing a fifth variant of the design.Figure 41 is a top plan view of the Blade for a Sickle showing a fifth variant of the design.Figure 42 is a bottom plan view of the Blade for a Sickle showing a fifth variant of the design.Figure 43 is an isometric view from the top and one side of the Blade for a Sickle showing a sixth variant of the design.Figure 44 is a first side elevational view of the Blade for a Sickle showing a sixth variant of the design.Figure 45 is a second side elevational view of the Blade for a Sickle showing a sixth variant of the design.Figure 46 is a front elevational view of the Blade for a Sickle showing a sixth variant of the design.Figure 47 is a rear elevational view of the Blade for a Sickle showing a sixth variant of the design.Figure 48 is a top plan view of the Blade for a Sickle showing a sixth variant of the design.Figure 49 is a bottom plan view of the Blade for a Sickle showing a sixth variant of the design.Figure 50 is an isometric view from the top and one side of the Blade for a Sickle showing a seventh variant of the design.Figure 51 is a first side elevational view of the Blade for a Sickle showing a seventh variant of the design.Figure 52 is a second side elevational view of the Blade for a Sickle showing a seventh variant of the design.Figure 53 is a front elevational view of the Blade for a Sickle showing a seventh variant of the design.Figure 54 is a rear elevational view of the Blade for a Sickle showing a seventh variant of the design.Figure 55 is a top plan view of the Blade for a Sickle showing a seventh variant of the design.Figure 56 is a bottom plan view of the Blade for a Sickle showing a seventh variant of the design.